What is the realistic accuracy & precision of typical consumer MPG calculations (tripmeter miles/pump gallons)

[Mad Roger]

What is a realistic accuracy & precision of typical MPG measurements when
measured by the consumer using the typical method of dividing their
tripmeter miles by the gas-pump gallons during fillup?

[Vic Smith]

Close to 100% accuracy if done right. I've done it on long trips. But MPG will vary
depending on terrain, weather, wind direction, stop-and-go traffic, etc.
So if you want "true" MPG for your car, you have to do it for the life of the car.
Once you do it initially, it's kind of pointless to do again except to satisfy your
curiosity.

[cl...@snyder.on.ca]

On Thu, 20 Jul 2017 12:08:33 -0000 (UTC), Mad Roger
<roge...@yahoo.com> wrote:

Repeatabilty is terrible. Accuracy can be pretty good over multiple
tanks. Can be pretty good even on single tanks IF there is a way to
ensure the tank is always filled to EXACTLY the same point (like a
level in the fill - tube, with the vehicle parked at EXACTLY the same
place for each fill-up). Relying on the auto-shutoff of the pump can
cause variance of several liters per fillup.

[Frank]

My year old car gives constant mileage and appears very accurate when I
calculate based on fill-up. It even has a moving mileage meter going up
to 80 mpg when just coasting down hill or maybe 10 mpg going up hill.
I recall many times on long trips mileage varying all over the map
probably because I was filling up at different stations.

[Steve W.]

IF the odometer is accurate and you do the math out to the 10ths of a
gallon the pump shows it can be VERY accurate.

--
Steve W.

[Steve W.]

Vehicle position or the auto shut off point won't make any difference.
You read the amount of fuel you pumped off the pump itself. The only
real issue is odometer accuracy. That can vary with tire size variations
and factory calibration.

--
Steve W.

[John-Del]

On Thursday, July 20, 2017 at 9:42:15 AM UTC-4, Steve W. wrote:
> cl...@snyder.on.ca wrote:
> > On Thu, 20 Jul 2017 12:08:33 -0000 (UTC), Mad Roger
> > <roge...@yahoo.com> wrote:
> >
> >> What is a realistic accuracy & precision of typical MPG measurements when
> >> measured by the consumer using the typical method of dividing their
> >> tripmeter miles by the gas-pump gallons during fillup?
> > Repeatabilty is terrible. Accuracy can be pretty good over multiple
> > tanks. Can be pretty good even on single tanks IF there is a way to
> > ensure the tank is always filled to EXACTLY the same point (like a
> > level in the fill - tube, with the vehicle parked at EXACTLY the same
> > place for each fill-up). Relying on the auto-shutoff of the pump can
> > cause variance of several liters per fillup.
>
> Vehicle position or the auto shut off point won't make any difference.
> You read the amount of fuel you pumped off the pump itself.

That's only assuming you started with a literally empty tank and ran it empty.

[pf...@aol.com]

There is a difference between Accuracy and Precision.

Keeping it simple and consistent: Run the filler to the first click. Do NOT round to the nearest whole number. Note the miles.

Run the filler to the first click. Do the math.

You will be within a percent or two, certainly far more accurate than your driving is consistent. Meaning, that if you do this over 50 gallons of fuel, or so (about 2,410 miles in my case), you will have a single-decimal average MPG that is good enough for most uses.

My on-board computer calculates electric miles, regenerative miles and gasoline use with each trip at two decimal places. Doing check-math on paper just for giggles, my one-decimal calculation was dead-on via the method described - based on total miles driven and total gasoline used.

Peter Wieck
Melrose Park, PA

[Ralph Mowery]

In article <okqbo5$c17$1...@dont-email.me>, csr...@NOTyahoo.com says...

If you just do it one time, you can not be sure you put in the same
ammount of fuel that was taken out.

If you keep a running total of the ammount of miles and fuel over
several tank fulls , the ammount of fuel will sort of average out.

Say you park so the back of the car is up hill and you fill the tank.
Go a number of miles and fill up again. This time the back of the car
is down hill. You may burn out 15 gallons, but only put back in 14
gallons. Ot it could be the other way around and you burn 14 gallons,
but only put back in 13 gallons. From tank to tank full there could be
a large variation. Over many tanks, the variation will average out to a
lessor error.
After say 10 tanks used you only have to contend with one or two errors
caused by the exact ammount of fuel put in the tank. Probably just the
last tank full would be where the error would come in. So instead of 1
gallon of error like the example above, you would have about .1 gallon
of error if the pumps are correct, which they should be.

[Vic Smith]

If you're getting MPG on a long trip, you only fill the tank all the way when you start and
when you finish the trip. All the other gas stops you go by the meter reading.
Very little room for error if you write down the meter reading.

[Mike Coon]

In article <okq6g0$409$1...@news.mixmin.net>, roge...@yahoo.com says...

>
> What is a realistic accuracy & precision of typical MPG measurements when
> measured by the consumer using the typical method of dividing their
> tripmeter miles by the gas-pump gallons during fillup?

Trip meter miles depends on circumference of driving wheels. I know my
speedo closely matches readings of roadside radar displays or my GPS, so
I guess trip meter miles will be accurate too.

[pf...@aol.com]

fly shit to the left, pepper to the right.

Guys and gals - *EVERYTHING* depends on the accuracy of the odometer, the variation in the operating diameter of the tires, the amount of friction involved (LRR tires vs. AT tires as one example), even tire pressure. Then, add head or tail winds, external temperature, type and age of road surface, number of people in the car, number of dead McDonald's wrappers in the back, and so on and so forth.

If one wants five-decimal precision (that is also accurate) then one will need more tools than a commercial gas-pump reading and a simple odometer reading. However, if one wishes simply one-decimal accuracy, the problem is trivial and needs very little analysis-in-depth.

And, whether the car is parked on a hill pointing up, or pointing down, if it is being fueled at a legal, code-compliant fuel station, the flatness at the pump islands is regulated. Despite the many reaches to the contrary, this ain't nohow rocket science! It barely rises to slide-rule requirements... .

[cl...@snyder.on.ca]

On Thu, 20 Jul 2017 09:42:10 -0400, "Steve W." <csr...@NOTyahoo.com>
wrote:

No, steve. You are wrong. The amount of fuel you put in is the amount
you can squeeze into the empty portion of the tank. The amount you
used is the amount that used to be in the tank. You need to fill it to
the exact same point each time to get an accurate reading. You may
have filled your 72 liter tank to only 71 liters the last time you put
in 50 liters to fill the tank. Now, at a different station, with
different levels, you may squeeze in 73, or only 68. COSISTANCY is the
key - and where most will fall down, because, like you, they just
don't REALLY understand.

[cl...@snyder.on.ca]

On Thu, 20 Jul 2017 10:57:06 -0400, Ralph Mowery
<rmower...@earthlink.net> wrote:

If the first and last are identical, none of the others matter. The
difference of 1, or 10 liters spread across many tanks becomes , more
or less, just noise. On the short term, like 1 tank, it can be a
pretty large percentage of error.

[Frank]

You buy gas by the gallon and mileage is miles per gallon. Summer gas
has higher density so you get more gas by weight for your money but it
costs more per gallon in the summer. Sounds like you can't win.

[Mad Roger]

On Thu, 20 Jul 2017 17:32:39 +0100,
Mike Coon wrote:

>> What is a realistic accuracy & precision of typical MPG measurements when
>> measured by the consumer using the typical method of dividing their
>> tripmeter miles by the gas-pump gallons during fillup?
>
> Trip meter miles depends on circumference of driving wheels. I know my
> speedo closely matches readings of roadside radar displays or my GPS, so
> I guess trip meter miles will be accurate too.

Every reading a mom and pop does has inaccuracies that, I posit, are
tremendously higher than most people seem to think they are (at least most
people who quote mpg figures with decimal places in them).

Most people have a tripmeter reading and a gas pumpmeter reading.
Where they fill the tank and reset the tripmeter before driving away.

I can't find any reliable source that says what the accuracy or
repeatability of that mom-and-pop tripmeter/pumpmeter calculation, but
basic logic dictates that the errors compound such that there is likely
(IMHO) no way to get anywhere near decimal-point accuracy, and worse,
probably plus or minus 1 mpg is the closest anyone can get in terms of
repeatability and precision.

Even the EPA's $360,000 machine only claims plus or minus 2% of the
indicared reading. I can't find where I got the notion that a mom and pop
can't possibly get closer than about 4% with a tripmeter/pumpmeter mpg
calculation - but I'm still seeking those numbers as we speak.

[dpb]

On 07/21/2017 11:51 AM, Mad Roger wrote:
...

> Most people have a tripmeter reading and a gas pumpmeter reading.
> Where they fill the tank and reset the tripmeter before driving away.
>
> I can't find any reliable source that says what the accuracy or
> repeatability of that mom-and-pop tripmeter/pumpmeter calculation, but
> basic logic dictates that the errors compound such that there is likely
> (IMHO) no way to get anywhere near decimal-point accuracy, and worse,
> probably plus or minus 1 mpg is the closest anyone can get in terms of
> repeatability and precision.

...

Why do errors compound in your view?

And, it depends on what you mean in terms of accuracy -- in terms of
absolute one needs to know the calibration error of the odometer; most
folks are satisfied to just assume it's close enough for the purpose.

If you look at simply a single fillup, it's not unreasonable to expect a
few tenths of a gallon difference between the first fillup level and the
subsequent; if you try it on shorter distances than a full tank then the
fractional error goes up.

OTOH, if one keeps track over longer periods of multiple fillups and
take some care to use the same filling pattern and only fills up after
using near the full tank capacity, then over time plus/minus targets
_will_ tend to cancel out and I have no qualms in believing a relative
performance number in the 0.1 mpg can be determined.

As noted, I've done this on long trips a number of times (generally on
first trip or so with a new vehicle, either actually new (rare) or (most
often) new to me) just to see how it compared with previous and have had
quite good comparisons on recent ones with the computer-computed
results. These would be over total distances of 1500 to 2000 miles, not
just 20 miles test runs.

--

[rickman]

+1

I measure my gas mileage on every fillup. I get 19 to 20 MPG every fill
unless I do a lot of around town driving. Very consistent. I watch it to
see if it drops off which would mean something is wrong.

--

Rick C

[Vic Smith]

On Fri, 21 Jul 2017 16:51:17 -0000 (UTC), Mad Roger <roge...@yahoo.com> wrote:


>
>Even the EPA's $360,000 machine only claims plus or minus 2% of the
>indicared reading. I can't find where I got the notion that a mom and pop
>can't possibly get closer than about 4% with a tripmeter/pumpmeter mpg
>calculation - but I'm still seeking those numbers as we speak.

The EPA doesn't have the time to do accurate MPG numbers Plus/Minus 2% is good enough for
the EPA. I'm sure my "mom and pop" MPG number are more accurate than that.
But so what? The MPG I get depend on the driving circumstances.
For instance, I've measured my MPG on a number of cars on 3000 mile round trips to Florida.
I can tell you the EXACT total MPG I got on those 3000 mile trips because I carefully noted
the exact metered amount of gas I used, and I verified the odometer accuracy using mile
markers. The only real useful thing that gives me is my MPG for the entire trip.
That includes local traffic when getting off the highway, and my travels at my destination.
But I know my approximate MPG at steady highway speed because I sometimes do tank to tank
calculations by filling to the filler tube. That too is an EXACT calculation, but is still
only approximate MPG because maybe the terrain and weather may vary.
So before you ask about "accurate MPG" you have to define what that is.

[Mad Roger]

On Fri, 21 Jul 2017 12:42:08 -0500,
dpb wrote:

> Why do errors compound in your view?

It's like a chain is no stronger than the weakest link.

No calculated result can be better than the worse inaccuracy.

> And, it depends on what you mean in terms of accuracy -- in terms of
> absolute one needs to know the calibration error of the odometer; most
> folks are satisfied to just assume it's close enough for the purpose.

Accuracy, precision, and sigfigs are standard terms:
http://www.chem.tamu.edu/class/fyp/mathrev/mr-sigfg.html
Accuracy: how closely a measured value agrees with the correct value.
Precision: how closely individual measurements agree with each other.
Sigfigs: accuracy is no better than the least accurate measurement.

By way of off-the-cuff example, if the accuracy of the odometer is to the
billionth of a mile and the accuracy of the pump gallons is to the
billionth of a gallon, but the accuracy of the fillup is plus or minus one
gallon, then the resulting mathematical (division or multiplication)
accuracy can be no better than plus or minus one gallon.

> If you look at simply a single fillup, it's not unreasonable to expect a
> few tenths of a gallon difference between the first fillup level and the
> subsequent; if you try it on shorter distances than a full tank then the
> fractional error goes up.

A single fillup will never suffice.

We're trying to compare a MPG *change* between two situations, so, by
definition, there _must_ be (at the very least!) /two/ separate
calculations.
+ Calculation before the change (say, smaller tire/wheel diameter)
+ Calculation after the change (say, larger time/wheel diameter)

> OTOH, if one keeps track over longer periods of multiple fillups and
> take some care to use the same filling pattern and only fills up after
> using near the full tank capacity, then over time plus/minus targets
> _will_ tend to cancel out and I have no qualms in believing a relative
> performance number in the 0.1 mpg can be determined.

That's not necessarily true, because it depends on the understimations
balancing out the overestimations, but I'm not going to quibble that more
calculations done over time are likely going to randomize the precision and
accuracy fluctuations over time.

While I will not quibble with your statement (because I essentially agree
with you), I can point out that your speedometer can be consistently wrong
in the same direction in either precision or accuracy, in which case it's
*not* going to balance out over time. It will be consistently wrong, over
time.

But, let's not quibble about that because we both can assume that, for our
purposes, the randomization of measurement results will be half the time
underestimating and the other half the time overestimating - such that they
could balance out.

> As noted, I've done this on long trips a number of times (generally on
> first trip or so with a new vehicle, either actually new (rare) or (most
> often) new to me) just to see how it compared with previous and have had
> quite good comparisons on recent ones with the computer-computed
> results. These would be over total distances of 1500 to 2000 miles, not
> just 20 miles test runs.

Nobody yet, and even not me, has supported a claim for any better accuracy
than my presumed plus or minus one mile per gallon using the standard
mom-and-pop test of dividing the number of miles driven based on the
tripmeter reading by the pump indication of gallons used to fill back up to
a presumed similar previous starting point of amount of fuel consumed.

Remember, the resulting accuracy can't possibly be better than the least
accurate measurement.

[dpb]

On 07/21/2017 2:05 PM, Mad Roger wrote:
...

> Remember, the resulting accuracy can't possibly be better than the least
> accurate measurement.

On a _point_ estimate, yes.

The point I'm making is that it is the _total_ fuel consumed over the
total distance; the changes in hitting the target level on a
tank-by-tank basis goes away for all excepting the last tank as it
doesn't matter in the total. So, if you miss by 0.1 gal on the one
tank, yeah, that roughly will translate to 0.1 on the mpg number. But,
over the 9 tanks prior to the tenth and last, it doesn't matter; it was
all used and so the 0.1 gal error on the last is only a tenth of the
size on the overall as it was on the first.

So, over a time, you can get quite precise estimates this way.

As noted, the bias in odometer calibration is a bias, yes, but presuming
there's not a reason it is getting worse with time it's not compounding,
it just makes a percentage difference in the computed result.

--

[Mad Roger]

We're trying to compare a MPG *change* between two situations:

+ Calculation before the change (say, smaller tire/wheel diameter)
+ Calculation after the change (say, larger time/wheel diameter)

If the change itself causes, say, a 1 mpg difference, but if our
measurement accuracy is, say, plus or minus 1 mpg, then we'll never see a
measurable difference between the two test runs.

Even if we run ten thousand test runs, we'll still never see a
statistically valid difference, even though the 1 mpg difference is
actually there.

We can't measure any better than our accuracy and repeatability allows.

The factors, I think, are accuracy, precision, repeatability, and, since
multiplication/division is involved, each offset worsens the results.

Without answering these questions, nobody, yes, not even you, can say you
have an "exact" number, and, I posit, that you can't even get remotely
close to exact, using the standard mom-and-pop tripmeter/pumpmeter method.

+ Tripmeter accuracy is what in the average car over a 300-mile tank?
+ Owners ability to "match" the previous level of fuel is what?
+ Gas station pump reading accuracy is what?

[Mad Roger]

On Fri, 21 Jul 2017 14:20:43 -0500,
dpb wrote:

> On a _point_ estimate, yes.
>
> The point I'm making is that it is the _total_ fuel consumed over the
> total distance; the changes in hitting the target level on a
> tank-by-tank basis goes away for all excepting the last tank as it
> doesn't matter in the total. So, if you miss by 0.1 gal on the one
> tank, yeah, that roughly will translate to 0.1 on the mpg number. But,
> over the 9 tanks prior to the tenth and last, it doesn't matter; it was
> all used and so the 0.1 gal error on the last is only a tenth of the
> size on the overall as it was on the first.
>
> So, over a time, you can get quite precise estimates this way.
>
> As noted, the bias in odometer calibration is a bias, yes, but presuming
> there's not a reason it is getting worse with time it's not compounding,
> it just makes a percentage difference in the computed result.

Your multiple-runs argument only holds water for both random accuracy and
random precision, but not if one is random and the other is not.

For example, I think it's well known that most speedometers read high
*most* of the time (at least that's my understanding - but I could look
that up if you question that assertion).

Assuming that assertion is close to correct, let's say they read high by
about 5% accuracy most the time (just to make a point), where the precision
is about plus or minus 1%.

Notice the accuracy is *always* high while the precision is random around a
set point.

http://www.chem.tamu.edu/class/fyp/mathrev/mr-sigfg.html
Accuracy: how closely a measured value agrees with the correct value.
Precision: how closely individual measurements agree with each other.

If the speedo reads high by 5% all the time, whether you measure your speed
once or if you measure your speed a billion times, you'll never any closer
to the right speed than 5% plus or minus 1%.

In repeatability, the gauge may give you different figures within + or - 1%
of that 5%, which is only to say that the speed will be consistently
reading from 4% to 6% higher than the actual speed.

But a billion test runs won't get you any better than that, all of which
are at least 4% off from the "correct" measurement (in the example).

My point is that a billion test runs only randomizes that which is random.

[root]

Mad Roger <roge...@yahoo.com> wrote:
>
> We're trying to compare a MPG *change* between two situations:
> + Calculation before the change (say, smaller tire/wheel diameter)
> + Calculation after the change (say, larger time/wheel diameter)
>

Putting different size wheels on the rear will affect the mileage
measurement apart from the mpg, so you will have to correct
the miles measurement before computing mpg. Smaller wheels => higher
miles for the same real distance. You will have to take into
account how you drive with the wheel change. If you maintain
the same real speed for smaller wheels your engine will be
turning over faster than before. Driving at the same speedometer
speed with smaller wheels reduces the load on the engine.

As a somewhat off-topic point, manifold vacuum is directly related
to instantaneous mpg. It is relatively easy to install a vacuum
gauge in the driver's compartment.

[rickman]

You obviously don't check your mileage very often. I do and seldom see even
1 MPG difference. I get high 19 or low 20 MPG on 19 out of 20 tanks. I
think the idea of uneven filling of the tank is a red herring. I can't
remember the last time I saw a gas station on a slope.

Bottom line is try it and see. I expect the major factor in MPG variation
is actual MPG variation from driving a different mix of town and highway
driving.

--

Rick C

[dpb]

On 07/21/2017 5:05 PM, Mad Roger wrote:
...

> But a billion test runs won't get you any better than that, all of which
> are at least 4% off from the "correct" measurement (in the example).
>
> My point is that a billion test runs only randomizes that which is random.

We'd already thrown the mileage calibration bias out as being simply
that. It can be compensated for by comparison over set measured course
and recording the offset. Red herring for the discussion.

The point I'm making is that it doesn't matter at all about whether
there's any random error in the fillup on individual tanks at all on the
intermediary answers--yes, they may have some fluctuation owing to
inconsistent fillup, but one can assume the pump is accurate since
they're checked by the State weights and measures folks on a regular
basis. So, all the fuel that went in went out in accumulating the miles
and it didn't matter how much went in on each individual measurement at
all in the end--it's the total. Only that random error on the final
fillup when you make the calculation at the end does that error enter in
-- and it becomes quite small by then in comparison to the total.

And, if one computes the mean of the billion trials, the error in the
mean is quite small even if the random error in each trial is sizable.

--

[Mad Roger]

On Fri, 21 Jul 2017 18:39:37 -0400,
rickman wrote:

> You obviously don't check your mileage very often. I do and seldom see even
> 1 MPG difference. I get high 19 or low 20 MPG on 19 out of 20 tanks. I
> think the idea of uneven filling of the tank is a red herring. I can't
> remember the last time I saw a gas station on a slope.

I don't think I've ever seen a gas station on a slope.
However ...
The errors in the calculation stem from errors that nobody seems to know
what they are, which means nobody knows what they're talking about.

Assuming the tripmeter/pumpmeter calcultion is the method used,
+ A tripmeter of 300 miles is neither accurate nor precise
+ A pumpmeter of 20.25 gallons is likely relatively accurate & precise
+ Matching fuel level in the tank isn't even close to accurate nor precise

Any one measurement (either miles or gallons alone) can only be as accurate
and precise as the worst measurement, while the miles/gallons calculation
compounds those inaccuracies and imprecisions (in loss of sig figs).

I think most of us would probably assume the pumpmeter is the most accurate
and the most precise, but the other two measurements aren't even close to
accurate or precise.

What matters is how accurate & precise is a 300 mile tripmeter reading?
And how accurate and precise is the match to the previous fuel level?

I posit that the best you can do, overall, after running the calculation,
is something like plus or minus about 1 mile per gallon such that 20 mpg is
actually anywhere from 19 to 21 miles per gallon actual.

What I'm seeking is data that tells us the three main questions that must
be answered for anyone to say that my hypothesis is even close to being
right or wrong:

+ How accurate & precise is a reading of 300 miles on a typical tripmeter?
+ How accurate & precise is a reading of 20.25 gallons on a gas pump?
+ How accurate & precise is the matching of the prior fuel level done?

No calculation can do better than the worst measurement, and worse, errors
compound when you multiply or divide.

> Bottom line is try it and see. I expect the major factor in MPG variation
> is actual MPG variation from driving a different mix of town and highway
> driving.

But that's the kind of things we're looking for, which is why the minimum
number of calculations possible is two, since you have to have a "before"
situation and an "after" situation.

For example, if the change that you are testing causes about 1 mile per
gallon decrease in fuel economy overall (but which isn't linear), but if
your calculations are no better than plus or minus 1 mile per gallon in
accuracy or precision, then you'll never even see the very real difference
because it will be unmeasurable given the plus or minus 1 mile per gallon
typical accuracy and precision that I posit the typical mom-and-pop
tripmeter/pumpmeter calculation provides.

But there's no sense in talking about *any* of that, if we don't know the
answer to these three questions.
+ How accurate & precise is a reading of 300 miles on a typical tripmeter?
+ How accurate & precise is a reading of 20.25 gallons on a gas pump?
+ How accurate & precise is the matching of the prior fuel level?

[Mad Roger]

On Fri, 21 Jul 2017 18:39:37 -0400,
rickman wrote:

> You obviously don't check your mileage very often. I do and seldom see even
> 1 MPG difference. I get high 19 or low 20 MPG on 19 out of 20 tanks. I
> think the idea of uneven filling of the tank is a red herring. I can't
> remember the last time I saw a gas station on a slope.

> Bottom line is try it and see. I expect the major factor in MPG variation
> is actual MPG variation from driving a different mix of town and highway
> driving.

[Vic Smith]

On Fri, 21 Jul 2017 21:51:42 -0000 (UTC), Mad Roger <roge...@yahoo.com> wrote:

> On Fri, 21 Jul 2017 13:31:01 -0500,
> Vic Smith wrote:
>

>> So before you ask about "accurate MPG" you have to define what that is.
>
>We're trying to compare a MPG *change* between two situations:
>+ Calculation before the change (say, smaller tire/wheel diameter)
>+ Calculation after the change (say, larger time/wheel diameter)
>
>If the change itself causes, say, a 1 mpg difference, but if our
>measurement accuracy is, say, plus or minus 1 mpg, then we'll never see a
>measurable difference between the two test runs.
>

Under just moderately controlled conditions, 1 MPG is actually a significant amount, and
would be easily detectable using your mom and pop method, assuming the mom and pop
competently applied the method.

>Even if we run ten thousand test runs, we'll still never see a
>statistically valid difference, even though the 1 mpg difference is
>actually there.
>
>We can't measure any better than our accuracy and repeatability allows.
>
>The factors, I think, are accuracy, precision, repeatability, and, since
>multiplication/division is involved, each offset worsens the results.
>
>Without answering these questions, nobody, yes, not even you, can say you
>have an "exact" number, and, I posit, that you can't even get remotely
>close to exact, using the standard mom-and-pop tripmeter/pumpmeter method.
>
>+ Tripmeter accuracy is what in the average car over a 300-mile tank?
>+ Owners ability to "match" the previous level of fuel is what?
>+ Gas station pump reading accuracy is what?

I never used the tripmeter for MPG, because I never bothered testing them with mile
markers.
Matching gas level is trivial - and it only has to done at the beginning and end of the
trip.
Gas station pumps - I assume they are accurate, and can't control that anyway.
I'm confident that my measurements are accurate to within .1 MPG.
Because I don't care about .135867 on the total. I round it down to .1
That's pretty "exact." Repeatability is meaningless when measuring MPG, unless you're
driving on a covered track, with a temperature controlled environment.
On every trip the MPG can vary.
BTW, I don't disagree that perfect measurement of MPG in unattainable. Perfection is
impossible even under lab conditions.
But you too easily discount "mom and pop" MPG calculations.

[Mad Roger]

On Fri, 21 Jul 2017 18:05:52 -0500,
dpb wrote:

> We'd already thrown the mileage calibration bias out as being simply
> that. It can be compensated for by comparison over set measured course
> and recording the offset.

I agree with you that the tripmeter calculation is inaccurate to some
degree, for which there are ways of "calibrating" the equipment.

> Red herring for the discussion.

The answer to the question depends on only 3 factors, I think.

Given these three factors are critical to answer the question, I think
everyone is talking out of their ass (including me) if they can't answer
these three questions to validate their own thought process:

+ How accurate & precise is a reading of 300 miles on a your tripmeter?
+ How accurate & precise is a reading of 20.25 gallons on the gas pump?

+ How accurate & precise is the matching of the prior fuel level?

I posit both the tripmeter and the previous-fill-level measurements suck.
How much to they suck?

I don't know.

I would not be surprised if they suck so badly that the end result is a
calculation which is plus or minus 1 mile per gallon in either accuracy or
repeatability.

> The point I'm making is that it doesn't matter at all about whether
> there's any random error in the fillup on individual tanks at all on the
> intermediary answers--yes, they may have some fluctuation owing to
> inconsistent fillup, but one can assume the pump is accurate since
> they're checked by the State weights and measures folks on a regular
> basis.

While it will be useful to know what the accuracy and precision
(repeatability) of the pump is, I think we can all assume that the pump is
within something like (at least) plus or minus a few percent of what it
reads.

But that number can be accurate to a billionth of a gallon, and it still
would be meaningless if the fill level was off by plus or minus a gallon
because the accuracy of any one measurement is only as good as the worst
measurement and the accuracy of the final calculation (when multiplication
adn division are involved) compounds inaccuracies.

Anyway, all the words are moot if we don't know the answer to 3 questions:
+ How accurate & precise is a reading of 300 miles on a your tripmeter?
+ How accurate & precise is a reading of 20.25 gallons on the gas pump?

+ How accurate & precise is the matching of the prior fuel level?

> So, all the fuel that went in went out in accumulating the miles
> and it didn't matter how much went in on each individual measurement at
> all in the end--it's the total. Only that random error on the final
> fillup when you make the calculation at the end does that error enter in
> -- and it becomes quite small by then in comparison to the total.

Am I correct to understand that you are saying if you go only 300 miles on
one tank, then the fill-level inaccuracy is (say) plus or minus 1 gallon
per tank; but if you go 3,000 miles (obviously on multiple tanks), that the
fill-level inaccuracy is one tenth of that plus or minus one gallon per
tank?

> And, if one computes the mean of the billion trials, the error in the
> mean is quite small even if the random error in each trial is sizable.

As long as the error is random (i.e., in both directions of the true
answer).

[Mad Roger]

On Fri, 21 Jul 2017 18:22:08 -0500,
Vic Smith wrote:

> I'm confident that my measurements are accurate to within .1 MPG.

Assuming one tank is about 300 miles and about 20.25 gallons read on any
one pump, on a long trip of ten times that, you still can't be confident of
that 1/10th of a mpg unless you know the answer to these questions.

+ How accurate & precise is the combined reading of 3000 miles on your tripmeter?
+ How accurate & precise is the combined reading of 202.50 gallons on the 10 gas pumps?
+ How accurate & precise is the matching of the 1st & last fuel levels?

Remember the exammple of the speedometer, where it's *always* going to be a
few percent wrong, even if you drove a billion miles to try to "randomize"
out the errors.

[dpb]

On 07/21/2017 6:13 PM, Mad Roger wrote:
...

> + How accurate& precise is a reading of 300 miles on a typical tripmeter?

For a specific vehicle, it really doesn't matter to determine _changes_
in mpg for a given test condition (unless, of course, you're futzing
with the tire size in which case that would have to be compensated for
but is doable to pretty precise number by knowing the tire profiles or
simply doing the "measured mile" computation.

> + How accurate& precise is a reading of 20.25 gallons on a gas pump?

NIST tolerance is 6 in^3 in a 5 gal measure. AFAIK that's what all
state W&M departments use for their tolerance. A NIST document of
20,000 tested meters showed 0-mean normally distributed discrepancies at
about 90% bounds on the +/-6 number. The 6/5gal --> ~0.5%

> + How accurate& precise is the matching of the prior fuel level done?

That's entirely dependent upon the tester -- in older vehicles without
the emissions control folderol on the gas tank filler spout it was
pretty easy to be quite precise if one were trying. Now it would take
some doing, but I suspect if really cared, one could manage to get
pretty close to the same height.

> No calculation can do better than the worst measurement, and worse, errors
> compound when you multiply or divide.

...

But properly designed 'spearmints can cause cancellation of many sources
often and besides the simple "combination of error" the math of the
numbers also enters into the relative magnitude of the error on the
final result.

In this case, for example, the second can be effectively eliminated or
reduced significantly by simply taking multiple runs...then the actual
level on each intermediary measurement is immaterial because whether it
was high or low on any given case, the total amount of fuel is the
denominator and so the fractional error in it is much smaller owing to
the same presumed error in the last measurement.

--


--

[dpb]

On 07/21/2017 7:02 PM, dpb wrote:
...

>> + How accurate& precise is a reading of 20.25 gallons on a gas pump?
>
> NIST tolerance is 6 in^3 in a 5 gal measure. AFAIK that's what all state
> W&M departments use for their tolerance. A NIST document of 20,000
> tested meters showed 0-mean normally distributed discrepancies at about
> 90% bounds on the +/-6 number. The 6/5gal --> ~0.5%
>
>> + How accurate& precise is the matching of the prior fuel level done?

...

And remember that is the "shut 'er down" tolerance, not the average...as
noted, the most probable based on the NIST sample was in the +/-0 bin (<1).

I didn't quite recognize what the figure was yet when first looked at it
and had closed the link when I realized the significance so don't have
the actual numbers at hand...but the +/-6 number was quite a way out on
the tails of the distribution altho I don't know just precisely the
tails percentages.

And, actually while the report used "normal" in discussing the
distribution, it really wasn't normal as in bell-shaped, it was
symmetric and zero-mean, but the tail in each direction dropped off more
as hyperbolic than a normal--hence the tail percentages would actually
by somewhat lower than a real normal of same mean, standard deviation.

--

[dpb]

On 07/21/2017 6:30 PM, Mad Roger wrote:
...

> As long as the error is random (i.e., in both directions of the true
> answer).

What do you not understand about "random"?

And the mean is still the mean, whether it is zero or not.

--

[cl...@snyder.on.ca]

With my scanguage on my Ranger the calculated MPG and the MPG figured
out by me using a calculator and fuel volume vs mileage is generally
pretty darn close. If the ScanGuage says they injectors have passed
13.7 gallons, my fillup is generally somewhere within .1 to .2
gallons. The speedo and GPS are within less than 1 kph on speed at
100kph (62Mph), and the odo within about the same. This is after
making corrections over many tanks for the fuel volume adjustments.
Neither of my ancient machines has a built in "trip computer"

[cl...@snyder.on.ca]

Occaisionally on a longish trip I'll see how well I can drive for
economy - to see if I can better the last time I did that trip. This
is generally over pretty close to a full tank - and small differences
in driving technique can make a HUGE difference. So can a small change
in route.Or a difference in the wind. I've registered a good 25%
difference in mileage between 2 trips, both trying to squeeze the last
foot out of a liter of fuel. Round trip averages out the difference in
altitude.

[dpb]

On 07/21/2017 8:41 PM, cl...@snyder.on.ca wrote:
...

> With my scanguage on my Ranger the calculated MPG and the MPG figured
> out by me using a calculator and fuel volume vs mileage is generally
> pretty darn close. If the ScanGuage says they injectors have passed
> 13.7 gallons, my fillup is generally somewhere within .1 to .2
> gallons. The speedo and GPS are within less than 1 kph on speed at
> 100kph (62Mph), and the odo within about the same. This is after
> making corrections over many tanks for the fuel volume adjustments.
> Neither of my ancient machines has a built in "trip computer"

I find all that perfectly believable...the speedo on the Buick I noticed
on the last trip to NM was deadnuts on the wife's GPS. I presume if
that's the case the odo would be, too, altho I never actually checked on
it specifically.

Instrumentation is pretty good and pretty cheap to get pretty good for
ordinary measurements any more...electronics is a wunnerful help in many
ways. :)

As I've turned into old fogey, I've come to rely on the 'puter for such
info more than would've cared at 20. There's just only so many times
one really wants to get down on the ground and measure tire pressure
after 70... :( There's getting to be a lot of fluff besides the useful,
but the basic stuff is helpful.

--

[cl...@snyder.on.ca]

On Fri, 21 Jul 2017 13:31:01 -0500, Vic Smith

We'll never get that - he admits he's an "engineer"
1) Someone who gets excited obout things most other people don't care
about
2) Someone who solves a problem you didn't know you had, in a way you
don't understand.
3) The optimist sees the glass as half full. The pessimist sees the
glass as half empty. The engineer sees the glass as twice as big as it
needs to be


Q: What is the definition of an engineer?
A: Someone who solves a problem you didn't know you had in a way you
don't understand.

Q: When does a person decide to become an engineer?
A: When he realizes he doesn't have the charisma to be an undertaker.

Q: How can you tell an extroverted engineer?
A: When he talks to you, he looks at your shoes instead of his own.

Q: Why did the engineers cross the road?
A: Because they looked in the file and that's what they did last
year.

Q: How do you drive an engineer completely insane?
A: Tie him to a chair, stand in front of him, and fold up a road map
the wrong way.

Real Engineers consider themselves well dressed if their socks match
Real Engineers buy their spouses a set of matched screw- drivers for
their birthday.
Real Engineers wear moustaches or beards for "efficiency". Not
because they're lazy.
Real engineers have a non-technical vocabulary of 800 words.
Real Engineers think a "biting wit" is their fox terrier.
Real Engineers know the second law of thermodynamics - but not their
own shirt size.
Real Engineers say "It's 70 degrees Fahrenheit, 25 degrees Celcius,
and 298 degrees Kelvin" and all you say is "Isn't it a nice day"
Real Engineers don't find the above at all funny.

[cl...@snyder.on.ca]

Actually it is - because you only make ONE calculation. The
measurement accuracies do not change. The precision gets better,
because you are not using numerous mileage measurements that can only
be accurate to the closest 10th of a mile or KM (more accurate in the
metric system becuase a KM is smaller than a mile) The precision is
now 1/10km over 5000 km instead of 1/10 kn over 500, ten times.

Your accuracy on the fuel used is to the 10th of a liter (or gallon
if you are a Yank)- again more accurate with the metric system because
liters are smaller than gallons. So your accuracy is to the closest
10th of a liter 5 times - and the accuracy of the fillup is only the
closest you can get it ONCE instead of 5 times.

Accuracy of fuel used will be, at the very most, 5X 1/10th liter more
- that's plus half a liter over to -0 liter under over 365 liters at
20 miles per US Gallon that's within better than 1.5% (1.369) at the
outside.(assuming the calibration of the pumps is accurate - pumps are
calibration tested on a more or less regular basis - when I was "in
the business" we were still running imperial gallons for the most part
- the pumps were inspected and certified accurate to within 1/10
gallon in 5 gallons ( the closest the meter could read) IIRC when the
switch to metric was made, it was 1/10 of a liter in 20 liters (the
size of the test container remained virtually the same) so accuracy
improved by roughly a factor of 4.

If I kept track of the fuel mileage on my vehicle over a period of
50,000 km, the accumulated average fuel economy could be easily
calculated to within that percentage of error. ( I used to do that
when I ran a vehicle log for business purposes)

>
>While I will not quibble with your statement (because I essentially agree
>with you), I can point out that your speedometer can be consistently wrong
>in the same direction in either precision or accuracy, in which case it's
>*not* going to balance out over time. It will be consistently wrong, over
>time.

Or consistently right. If you KNOW the accuracy of the speedo, it is a
simple mathematical correction to achieve accuracy. The speedo may
vary in accuracy because it is an inductive coupling device on a
mechanical speedo, while the ODO will not vary as it is a direct
geared connection to the driven wheels. With electronic speedos and
odos, their calibration does NOT change. The only vatiance is tire
wear ( aproxematelt 3/8 inch difference in diameter of a , say, 24
inch diameter tire, over it's lifetime - and that wear is pretty
linear - so it is not rocket science to work in a correction for that
too if you want to be a very anal engineer.

>
>But, let's not quibble about that because we both can assume that, for our
>purposes, the randomization of measurement results will be half the time
>underestimating and the other half the time overestimating - such that they
>could balance out.

Or they could not - better to eliminate the randomization, or account
for it in calculating accuracy.

>
>> As noted, I've done this on long trips a number of times (generally on
>> first trip or so with a new vehicle, either actually new (rare) or (most
>> often) new to me) just to see how it compared with previous and have had
>> quite good comparisons on recent ones with the computer-computed
>> results. These would be over total distances of 1500 to 2000 miles, not
>> just 20 miles test runs.

I've averaged it over 240,000 miles - - -

>
>Nobody yet, and even not me, has supported a claim for any better accuracy
>than my presumed plus or minus one mile per gallon using the standard
>mom-and-pop test of dividing the number of miles driven based on the
>tripmeter reading by the pump indication of gallons used to fill back up to
>a presumed similar previous starting point of amount of fuel consumed.

I'm sure I could claim accuracy to closer than 1 MPG, but what good
would it do over 240,000 miles??????? (and how could you prove me
wrong?)

For COMPARISON testing, accuracy is not important - only precision
and repeatability. On my electric conversion I could compare one type
of tire to another by driving a given distance and route on one set,
measure the watt hours of charge used, and compare to a different set
of tires over the same route under the same conditions and KNOW how
much better "mileage" I would get with one tire over the other.

Modifying the tune on my '63 Valiant, or a customer's Celica, or
whatever - I could do a "before and after" run of 5 miles with my
calibration can and know, to the ounce, how much more or less fuel was
consumed over the same route, Using the "official" fuel mile tester I
could measure to the cc over a half liter - that's an accuravy of 1
part in 500, or 0.2 percent. If I had a "rolling road" I could repeat
the drive cycle accurately too - but I only had access to that at
trade school (a chassis dynamometer) Not as easy to do today with fuel
injected cars - but dynos are a LOT more common today than they were
40 to 45 years ago . . . and more programmable. If you know the cd of
a vehicle today, a computer simulation can run a vehicle over a
virtual course, correcting for ambient wind, changes in elevation,
accelleration (knowing mass of the car) -every conceivable condition
- to make direct case to case comparisons EXREMELY accurate. (and fuel
measurement technology has advanced so it's very simple to very
accurately neasure the amount of fuel consumed as well - and also get
very accurate measurements of instantaneous consumption - and with
strain guages even know EXACTLY how much horsepower is being delivered
to the road to figure out specific fuel consumption -

All stuff you "engineers" should understand.

>
>Remember, the resulting accuracy can't possibly be better than the least
>accurate measurement.

Which can vary from no better than a SWAG to pretty darn close, even
for the "mom and pop" or "hobyist" to EXTREMELY accurate for the
engineer.

[cl...@snyder.on.ca]

Whatever it is off, repeatability will be very close to 100%.
300 miles, repeatabilty to within less than 1/10 mile, or .2%

>+ Owners ability to "match" the previous level of fuel is what?

If he's as smart as the average fifth grader, within less than a cup
at the same station, in the same spot, Lets nake it within a pint -
that is 1/8 gallon in 15 - call it .8% on 15 gallons - - - so at
LEAST within 1%

>+ Gas station pump reading accuracy is what?

Again, repeatability within well under 1/10 gallon per fill (15
gallons) - about .6%??
Accuracy doesn't mean anything if the refill is at the same pump as
the initial fill.
Again, if you are in the "metric world" instead of the USA, accuracy
improves by a factor of almost 4 on the fuel measurements, and 1.6 on
the distance.


All of this accuracy can be accomplishes with NO special equipment.

Where the REAL fun comes in is duplicating the "drive cycle" - for the
average driver, on the average road, "virtually impossible". If you
have a closed course, and you are a VERY good and consistent driver, -
perhaps you can get within a REASONABLE approxemation (repeatability
over 5 runs, perhaps within less than 5%) - in open traffic, you'l be
doing VERY good to get repeatability better than within 10% on a short
run - over 300 miles, averaged over 5 runs (total 1500 miles) mabee
within 1% on open road, or 5% running with traffic.
Repeatability gets a LOT better over a short distance where fewer
variables are involved. (Start from the gas station, accellerate to
30MPH in 2 blocks, enter the "expressway" and accellerate to 60mph by
the first exit, maintain 60MPH to the 5th exit, slow down and exit the
highway, stop. pull away from stop and re-enter the highway going the
opposite direction, accellerate to 60 mph as you merge with traffic,
decellerate from the 4th exit to the third exit, return to the gas
station
Repeat.
Repeat
Repeat.

That kind of driving can be very repeatable.

Fill with gas, Drive 3 blocks to the highway, drive 100 miles to "the
city" Drive 5 miles across the city to restaurant, sports arena, or
place of work, then return

Repeat
Not much chance the 2 trips will be anywhere close. (I have had
variances of more than 25% between 2 trips between Waterloo and Barrie
Ontario over the same route, at the same time of day, and day of the
week (and even the same month) - (even when the overall travel time
was very close to the same for the round trip). generally WITHIN 10%
IS PRETTY DOABLE - AND AT 20mpg THAT'S +/- 1 mpg at best

If you are in open country with very little traffic, repeatability
gets MUCH better.
Driving back and forth from Flagstaff to Jeddito Arizona you could
likely repeat within well less than 1/4 MPG in the middle of the week
- or from Enid to Chickasha Oklahoma - or from Saskatoon to
Regina.(very little traffic variability)

[cl...@snyder.on.ca]

True of the speedometer, but NOT of the odometer. The odometer
repeatabilty is as close to 100% as you will get even with a cable
driven odometer. (it is a directly geared measuring device with ZERO
vatiability - X number of cable turns per mile from the day it's made
till the day it is scrapped ( generally 1000 turns per mile, but some
older cars were 600 turns per mile, some motorcylses 1450, etc - but
they never change) With electronic speedos and odos (virtually all
cars today less than 15 years old) repeatability is almost 100%.
Accuracy CAN be very close to 100% too, as on most cars under 10 years
old today, the speedometer can be accurately reprogrammed to the tire
diameter so repeatability is only affected by tire wear (mabee 3/8
inch in 24 over the life of the tire)

[cl...@snyder.on.ca]

On Fri, 21 Jul 2017 22:25:04 +0000 (UTC), root <NoE...@home.org>
wrote:

Directly related? but not necessarily 100% linearly related High
manifold pressure (low vacuum) means heavy load which means poor
mileage. The reverse is also true - but calibrating vacuum to MPG is
virtually impossible with any level of accuracy. It WILL give you a
good, better, worse indication though. Keep the vacuum up and you will
get better mileage.

[cl...@snyder.on.ca]

How many ingels can sit on the tip if a pin???
That's about how ridiculous this whole discussion is getting

[rickman]

Nearly all my driving is on secondary highways so I pretty much am driving
at pretty optimal speeds for mileage although there are some traffic lights,
they tend to be miles between stops. I have developed fuel efficient habits
so I nearly always squeeze every last MPG on my trips. I have a manual, so
I slip it out of gear and coast to lights and nearly always accelerate
gently. I leave a lot of room to the car in front so I can ease up on the
gas rather than hit the brakes. I think I am doing about as well as can be
expected all the time, so my mileage seldom varies unless I do more city
driving. High 19 or low 20 MPG, very consistent.

--

Rick C

[rickman]

Mad Roger wrote on 7/21/2017 7:13 PM:
> On Fri, 21 Jul 2017 18:39:37 -0400,
> rickman wrote:
>
>> You obviously don't check your mileage very often. I do and seldom see even
>> 1 MPG difference. I get high 19 or low 20 MPG on 19 out of 20 tanks. I
>> think the idea of uneven filling of the tank is a red herring. I can't
>> remember the last time I saw a gas station on a slope.
>
> I don't think I've ever seen a gas station on a slope.
> However ...
> The errors in the calculation stem from errors that nobody seems to know
> what they are, which means nobody knows what they're talking about.
>
> Assuming the tripmeter/pumpmeter calcultion is the method used,
> + A tripmeter of 300 miles is neither accurate nor precise

I don't know what you mean. I have checked my odometer against the markers
on the highway as well as against my GPS (I think the highway markers are
more accurate than the GPS). It is spot on with the current tires to 1% or
better. I had some larger tires at one point and it made the odometer read
a bit low, also the speedometer.

BTW, someone said something about one being accurate meant the other was
accurate and that is not necessarily true. My speedometer is mechanical and
so has a separate calibration factor. With the present tires it reads a bit
high, about 1 to 1.5 MPH at highway speeds. That one is harder to calibrate
than the odometer (which is pretty much on point) because it is hard to
maintain a constant speed for long enough to get an accurate reading even
with the cruise control. But with lots of readings I am pretty confident
these numbers are right.

So my odometer is accurate and precise.

> + A pumpmeter of 20.25 gallons is likely relatively accurate & precise

Of course it is. States inspect them at some point.

> + Matching fuel level in the tank isn't even close to accurate nor precise

I don't agree. I let the pump click off and then continue to pump for a
number of more clicks until it cuts off immediately. I always need to run
at least another fifteen miles before I am home so that is better part of a
gallon burned so I don't need to worry about the gas warming up and running
out of the tank. I believe this makes for very consistent fill ups.

My MPG results pretty well show the consistency of my measures.

> Any one measurement (either miles or gallons alone) can only be as accurate
> and precise as the worst measurement, while the miles/gallons calculation
> compounds those inaccuracies and imprecisions (in loss of sig figs).
>
> I think most of us would probably assume the pumpmeter is the most accurate
> and the most precise, but the other two measurements aren't even close to
> accurate or precise.

You know what happens when you assume... ;)

> What matters is how accurate & precise is a 300 mile tripmeter reading?
> And how accurate and precise is the match to the previous fuel level?
>
> I posit that the best you can do, overall, after running the calculation,
> is something like plus or minus about 1 mile per gallon such that 20 mpg is
> actually anywhere from 19 to 21 miles per gallon actual.

I think one time in nearly 20 years I got 22 MPG. I think I can count on my
fingers the times I got 21 MPG. These days with nearly all my driving on
the highway it is much less than 1 in 20 fills that I see less than 19 or
even 19.5 MPG. It is nearly always just under or just over 20 MPG, more
just under :-( If I were the dancing type I would have a little happy
dance when it actually is over 20 MPG, lol. It makes my day.

> What I'm seeking is data that tells us the three main questions that must
> be answered for anyone to say that my hypothesis is even close to being
> right or wrong:
>
> + How accurate & precise is a reading of 300 miles on a typical tripmeter?
> + How accurate & precise is a reading of 20.25 gallons on a gas pump?
> + How accurate & precise is the matching of the prior fuel level done?

I think the consistency of my MPG readings show how well each of these can
be measured. As you say, the pump is going to be dead on. Other than scale
error which can be calibrated out the odometer will be very good. Filling
your tank can be good as well. It's not like they design gas tanks to have
air pockets.

> No calculation can do better than the worst measurement, and worse, errors
> compound when you multiply or divide.
>
>> Bottom line is try it and see. I expect the major factor in MPG variation
>> is actual MPG variation from driving a different mix of town and highway
>> driving.
>
> But that's the kind of things we're looking for, which is why the minimum
> number of calculations possible is two, since you have to have a "before"
> situation and an "after" situation.
>
> For example, if the change that you are testing causes about 1 mile per
> gallon decrease in fuel economy overall (but which isn't linear), but if
> your calculations are no better than plus or minus 1 mile per gallon in
> accuracy or precision, then you'll never even see the very real difference
> because it will be unmeasurable given the plus or minus 1 mile per gallon
> typical accuracy and precision that I posit the typical mom-and-pop
> tripmeter/pumpmeter calculation provides.
>
> But there's no sense in talking about *any* of that, if we don't know the
> answer to these three questions.
> + How accurate & precise is a reading of 300 miles on a typical tripmeter?
> + How accurate & precise is a reading of 20.25 gallons on a gas pump?
> + How accurate & precise is the matching of the prior fuel level?

You don't need to know any of this specifically. You just need to measure
your fuel mileage and measure the accuracy and precision of the results.
Why do you care which of the three has what specific degrees of accuracy and
precision? You care about the accuracy and precision in the result and you
can measure that. Remember there are other factors as well that actually
impact your MPG from tank to tank. They will show up when trying to measure
any one influence so might as well calibrate them in too.

--

Rick C

[rbowman]

On 07/21/2017 07:55 PM, dpb wrote:
> Instrumentation is pretty good and pretty cheap to get pretty good for
> ordinary measurements any more...electronics is a wunnerful help in many
> ways.

When you consider how the old speedometers worked it's amazing they came
anywhere close to reality. I had a '60 Plymouth where the speedometer
looked like a red bar progressing across a horizontal display rather
than the usual needle. The guts were a tube about a foot long and an
inch and a half in diameter suspended in bearings and loaded with a
spiral spring. The mechanical cable from the tailshaft of the
transmission tweaked the tube with each revolution via a magnetic link.
It was an analog integrator with the spring controlling the tube's rotation.

The standard dial type was the same principal but the Chrysler engineers
went out of their way to be weird. That was also the era of the
pushbutton Torqueflite tranny and left handed lugnuts on one side.

A lot of modern speedometers are just as bizarre converting a perfectly
good digital pulse train to an analog voltage to drive a dial rather
than going straight digital.

But now

[rbowman]

On 07/21/2017 07:47 PM, cl...@snyder.on.ca wrote:
> Occaisionally on a longish trip I'll see how well I can drive for
> economy - to see if I can better the last time I did that trip.

I'm a fairly economical driver but on longish trips I'm more concerned
with getting there. 80 mph guarantees the fuel economy is going into the
dumpster.

[pf...@aol.com]

Once again: This horse is dead, skinned, flensed, tanned, jerked and dried. And whatever life it has clung to for these many repetitive, redundant and often ridiculous posts is based on the essential confusion between and conflation of "Accuracy" and "Precision". These do not mean the same thing, never have the same application, and seldom are on even parallel tracks when used properly.

When dealing with furlongs per bale, sufficient accuracy may be had using no more than 20% of the fingers of one normal human hand and the track markers. Miles per gallon, and kilometers per liter are quite similar, requiring only second-grade arithmetic to solve within repeatable limits. Full stop.

Peter Wieck
Melrose Park, PA

[rickman]

I forgot, I can tell the difference in fuel economy by driving 65 MPH rather
than 60. Driving at 65 very much (only about 1/3 of my trip allows that)
will assure that I only get 19 mpg rather than pushing 20.

There is a 10 mile stretch with only one traffic light and a posted speed
limit of 45 MPH. If I can get up to 50 so I'm solid in fifth gear my
mileage rocks.

--

Rick C

[Colonel Edmund J. Burke]

On 7/20/2017 5:08 AM, Mad Roger wrote:
> What is a realistic accuracy & precision of typical MPG measurements when
> measured by the consumer using the typical method of dividing their
> tripmeter miles by the gas-pump gallons during fillup?
>

Stuff like that doesn't concern a man of my means, but in a pinch I could measure my mileage knowing the actual distance to my nearest Denny's, America's Favorite Diner.
This is the "meat and potatoes" of the argument. All else is just fluff.

[Tekkie®]

cl...@snyder.on.ca posted for all of us...

+1

--
Tekkie

[rbowman]

I should look at the instantaneous readouts versus mph to see if the mpg
falls off gradually or if there is an efficiency sweet spot around
65-70. Except for around the cities the interstate speed limit in this
and some of the adjoining states is 80. Drive 65 at your own risk.

[Ed Pawlowski]

On 7/22/2017 1:38 PM, rbowman wrote:

>>
>
> I should look at the instantaneous readouts versus mph to see if the mpg
> falls off gradually or if there is an efficiency sweet spot around
> 65-70. Except for around the cities the interstate speed limit in this
> and some of the adjoining states is 80. Drive 65 at your own risk.
>

I tried that one day on a flat stretch so there would be little
variance. This was on my regular trip to work. Speed limit is 65. One
day I did 70, the next 65, then at 60 is was dicey, the next day I tried
55 for about 30 seconds and decided not to risk my life.

I forget the details, but 60 was better than 70 by a couple of mpg.
Problem is, I prefer driving 75. If I could get away with it I'd go 85+
but don't want to pay the fines.

[Mad Roger]

On Sat, 22 Jul 2017 00:46:50 -0400,
rickman wrote:

> So my odometer is accurate and precise.

I understand you because you're exactly the type of person that I had in
mind when I asked the question in the first place.

> I don't know what you mean. I have checked my odometer against the markers
> on the highway as well as against my GPS (I think the highway markers are
> more accurate than the GPS). It is spot on with the current tires to 1% or
> better.

Does your tripmeter have a decimal place and digits after that decimal
place?

> My speedometer is mechanical and so has a separate calibration factor.

The speedometer example was only brought in to point out that the vain hope
that averages result in better "accuracy" is patently false.

Mom-and-pop type of people actually believe that a speedometer reads even
close to accurately - and worse - some here propose the vain notion that
the more readings they take, somehow (magically?) the more accurate the
results will be.

A speedometer that reads high isn't going to result in more accurate
calculations even if you do a billion test runs.

>> + A pumpmeter of 20.25 gallons is likely relatively accurate & precise
>
> Of course it is. States inspect them at some point.

You don't seem to understand what accuracy and precision even mean.
Haven't you taken even one science lab course?

>> + Matching fuel level in the tank isn't even close to accurate nor precise
>
> I don't agree. I let the pump click off and then continue to pump for a
> number of more clicks until it cuts off immediately.

I'm not at all surprised about your concept of the fuel-level estimation,
and, in fact, you're exactly the mom-and-pop type person I was talking
about when I opened the thread.

I understand you.

> I always need to run
> at least another fifteen miles before I am home so that is better part of a
> gallon burned so I don't need to worry about the gas warming up and running
> out of the tank. I believe this makes for very consistent fill ups.

I'm sure you do believe that.

> My MPG results pretty well show the consistency of my measures.

I'm sure your MPG results support any theory you want them to support.
I believe you.

> You know what happens when you assume... ;)

You don't know how funny that statement was to me when I just read it now.

> I see less than 19 or even 19.5 MPG.

I bet you see that decimal place even though it's not in the tripmeter
estimation nor in the filllevel estimation.

You see, I understand you because you're the type of person I had in mind
when I asked the question.

> I think the consistency of my MPG readings show how well each of these can
> be measured.

I'm sure you do.

> As you say, the pump is going to be dead on.

Whoa! I never said the pump was "dead on" and anyone reading this thread
who thinks I think the pump is "dead on" would have completely
misunderstood everything else I said.

All I said was that the inaccuracies and imprecisions in the pump reading
are likely better than the otherwise astoundingly huge imprecision in the
fuel-fill level estimation and in the lesser inaccuracy of the tripmeter
estimation.

> Other than scale
> error which can be calibrated out the odometer will be very good.

Define "very good" please.

> Filling your tank can be good as well.

I'm sure you believe that filling the tank is "accurate" since you
calculate 19.5 miles per gallon and not something like 19.5 rounded up to
20 and then the error taken into account such that it's more likely
anywhere between 19 and 21 mpg than it is 19.5 mpg.

> It's not like they design gas tanks to have air pockets.

Actually, they do have air pockets.
Those air pockets change in size based on temperature & pressure & fill
level.

Even the fuel changes in density based on those parameters.

> You don't need to know any of this specifically.

Of course I don't. 19.5 mpg is all I need to know.
And if I change "something" which results in 19.7mpg, then of course, that
something was the cause. I understand. I really do.

> Why do you care which of the three has what specific degrees of accuracy and
> precision?

I care because when I do a calculation, my assumption is that 19.5mpg is
actually something closer to 19 to 21 mpg than it is to 19.5.

If the "change" I'm measuring is within that margin of error, then I can't
say anything about what that "change" was.

And, more importantly, neither can you.
Which is the entire point after all.

[dpb]

On 07/21/2017 8:30 PM, dpb wrote:
> On 07/21/2017 7:02 PM, dpb wrote:
> ...
>
>>> + How accurate& precise is a reading of 20.25 gallons on a gas pump?
>>
>> NIST tolerance is 6 in^3 in a 5 gal measure. AFAIK that's what all state
>> W&M departments use for their tolerance. A NIST document of 20,000
>> tested meters showed 0-mean normally distributed discrepancies at about
>> 90% bounds on the +/-6 number. The 6/5gal --> ~0.5%
>>

...

> And remember that is the "shut 'er down" tolerance, not the average...as
> noted, the most probable based on the NIST sample was in the +/-0 bin (<1).
>
> I didn't quite recognize what the figure was yet when first looked at it
> and had closed the link when I realized the significance so don't have
> the actual numbers at hand...but the +/-6 number was quite a way out on
> the tails of the distribution altho I don't know just precisely the
> tails percentages.
>
> And, actually while the report used "normal" in discussing the
> distribution, it really wasn't normal as in bell-shaped, it was
> symmetric and zero-mean, but the tail in each direction dropped off more
> as hyperbolic than a normal--hence the tail percentages would actually
> by somewhat lower than a real normal of same mean, standard deviation.

I got curious myself on what the numbers revealed and looked at the NIST
numbers again.

I computed an empirical cdf and compared it to normal...statistics from
the 20,036 observations are below:

>> [h,s]=cdfplot(x);
>> s
s =
min: -50
max: 146
mean: -0.0788
std: 3.7681
median: 0
mode: 0
>>

I then compared to normal on the same plot and as outlined above
N(mean,std) is too long-tailed on both ends in comparison. It turns out
that N(mean,std/1.5) is pretty close on both tails to about the +/- 6 point.


Anyway, from the above it's simple enough to get some pretty good
estimates of what pump volume errors one might expect...the table below
is from the empirical cdf NIST data...

P error(in^3)/5Gal error(%)
0.001 -22 -1.82
0.005 -9 -0.78
0.010 -8 -0.69
0.025 -6 -0.52
0.050 -5 -0.43
0.250 -2 -0.17
0.500 0 0
0.750 2 0.17
0.900 4 0.34
0.950 5 0.43
0.975 6 0.52
0.990 7 0.60
0.995 10 0.86
0.999 22 1.82

From the above, one can conclude the pump metering error small for all
except the extreme outlier pumps.

--

[cl...@snyder.on.ca]

The man is right You are wrong. You ASS U ME too much - and at the
risk of insulting the few GOOD engineers on the list, you OBVIOUISLY
are an "engineer", but not one I'd hire for a job. The job would come
in WAY over budget, WAY late, and would need to be completely redone
by techitians and technologists at great cost, or to save time and
money, completely decommissioned and scrapped - starting over with
someone who knew what thet were doing, and how to do it - engineer or
not.

[cl...@snyder.on.ca]

a whole lot of crap snipped

>
>I care because when I do a calculation, my assumption is that 19.5mpg is
>actually something closer to 19 to 21 mpg than it is to 19.5.
>
>If the "change" I'm measuring is within that margin of error, then I can't
>say anything about what that "change" was.
>
>And, more importantly, neither can you.
>Which is the entire point after all.

Roger, me lad - you wouldn't happen to be a britiah trained engineer,
now, would you?? In what discipline of engineering?

[Mad Roger]

On Sat, 22 Jul 2017 19:44:26 -0500,
dpb wrote:

> I got curious myself on what the numbers revealed and looked at the NIST
> numbers again.
>
> I computed an empirical cdf and compared it to normal...statistics from
> the 20,036 observations are below:
>

>> [2 quoted lines suppressed]

> s =
> min: -50
> max: 146
> mean: -0.0788
> std: 3.7681
> median: 0
> mode: 0

>> [1 quoted line suppressed]

>
> I then compared to normal on the same plot and as outlined above
> N(mean,std) is too long-tailed on both ends in comparison. It turns out
> that N(mean,std/1.5) is pretty close on both tails to about the +/- 6 point.
>
>
> Anyway, from the above it's simple enough to get some pretty good
> estimates of what pump volume errors one might expect...the table below
> is from the empirical cdf NIST data...
>
> P error(in^3)/5Gal error(%)
> 0.001 -22 -1.82
> 0.005 -9 -0.78
> 0.010 -8 -0.69
> 0.025 -6 -0.52
> 0.050 -5 -0.43
> 0.250 -2 -0.17
> 0.500 0 0
> 0.750 2 0.17
> 0.900 4 0.34
> 0.950 5 0.43
> 0.975 6 0.52
> 0.990 7 0.60
> 0.995 10 0.86
> 0.999 22 1.82
>
> From the above, one can conclude the pump metering error small for all
> except the extreme outlier pumps.

I love that you are the only one quoting actual numbers and not pulling
them out of your butt to answer the question!

But your numbers confuse me because they seem to be in cubic inches.
You also mentioned that metric pumps are more accurate, but that's
impossible, simply because the pump is as accurate as the pump can get,
which, we can assume, is a mechanical thing (and not a metric thing).

All you're saying is that a liter is four times smaller than a gallon so
the error is four times less for a given liter versus a given gallon but
that's not saying it's more accurate. It's just saying the volume is less
so the resulting error is less.

Anyways, can you just summarize what the error is for a typical USA pump in
gallons?

For a typical 20-gallon fill, how many gallons off can reality be, plus or
minus from the indicated reading on the pumpmeter?

[rbowman]

At under 70 my car usually is in the 35 mpg + range; at 80, it is more
like 32. I get even better mileage in Oregon with its 55 mph speed
limit. I also get bored out of my mind. There isn't a whole lot of
anything between Ontario and Bend but I figure as soon as I get up to a
decent speed a OSP cruiser will materialize from the sagebrush.

That stupid speed limit is the least of Oregon's problems.

[rbowman]

Engineering Management, I'm thinking.

[rbowman]

On 07/22/2017 08:40 PM, Mad Roger wrote:
> For a typical 20-gallon fill, how many gallons off can reality be, plus or
> minus from the indicated reading on the pumpmeter?

http://www.npr.org/templates/story/story.php?storyId=5304258

The article talks about Washington but most states have a similar
protocol. Pump 5 gallons of gas. 1 gallon is 231 cubic inches, so that
is 1155 cubic inches. The volume must be within 6 cubic inches or
roughly 0.5%. I'll let you do the math for 20 gallons.

http://billingsgazette.com/news/state-and-regional/montana/customers-rarely-shorted-at-the-gas-pump/article_3849a455-6151-515e-ae6a-2d65351736b4.html

Montana uses the same test. Note that he estimates 2 to 3% of the pumps
fail and have to be repaired and also says with normal wear the pumps
tend to dispense more than stated but some may dispense less. That's
where averaging over a number of tanks comes in unless you fill up at
the same pump at the same station every time. I certainly don't.

[cl...@snyder.on.ca]

You are the engineer, son of physics majors - figure it out!!!

>You also mentioned that metric pumps are more accurate, but that's
>impossible, simply because the pump is as accurate as the pump can get,
>which, we can assume, is a mechanical thing (and not a metric thing).

You fail to grasp the simple fact that a tenth of a liter is a whole
lot less than a tenth of a gallon???? Accuracy of READING the pump is
therefore about 4 times more accurate with a metric pump, because your
read error of +.1/-0 units is based on the much smaller unit.

>
>All you're saying is that a liter is four times smaller than a gallon so
>the error is four times less for a given liter versus a given gallon but
>that's not saying it's more accurate. It's just saying the volume is less
>so the resulting error is less.

and your engineer's understanding of accuracy does not equate to a
smaller error?????????

>
>Anyways, can you just summarize what the error is for a typical USA pump in
>gallons?

As good as Less than 1/10 of a percent according to the information
quoted, with a very few as bad as 1.82%. An american gallon is 128
fluid ounces, so 1.82% of 128 ounces is 2.23 ounces maximum error,
+/1, with most being within .5%, or 0.64 ounces per gallon

>
>For a typical 20-gallon fill, how many gallons off can reality be, plus or
>minus from the indicated reading on the pumpmeter?

The poorest pump checked in that data would be +/- 44.6 oz per 20
gallon tank - the average about +/- 12 ounces.
ASS U MEing the error is randomly distributed,around zero, your
chances of the error being anywhere CLOSE to even the 12 ounces is so
small as to be virtually insignificant unless you always used the same
pump - in which case it is totally immaterial if used for comparative
purposes.

For an engineer, you sure have a poor grasp of the concepts.

[Bill Vanek]

On Sat, 22 Jul 2017 22:12:26 -0600, rbowman <bow...@montana.com>
wrote:

>At under 70 my car usually is in the 35 mpg + range; at 80, it is more
>like 32. I get even better mileage in Oregon with its 55 mph speed
>limit. I also get bored out of my mind. There isn't a whole lot of
>anything between Ontario and Bend but I figure as soon as I get up to a
>decent speed a OSP cruiser will materialize from the sagebrush.
>
>That stupid speed limit is the least of Oregon's problems.

Where is their limit 55?

[rbowman]

The last time I was there US20, US395, and other 2 lane roads in eastern
Oregon. Apparently the raised it to 65 in March of 2016 but are rolling
it back in some places.

http://www.oregonlive.com/commuting/index.ssf/2016/06/oregon_rolls_back_speed_limit.html

but according to this the limit is now 70 on rural roads:

http://www.speed-limits.com/oregon.htm

70 on Rt. 20 would make a lot more sense if that is indeed what it is
now. I'm not planning to check it out personally though.

[Vic Smith]

On Sat, 22 Jul 2017 21:10:41 -0400, cl...@snyder.on.ca wrote:

>On Sat, 22 Jul 2017 23:42:25 -0000 (UTC), Mad Roger
><roge...@yahoo.com> wrote:
>
>> On Sat, 22 Jul 2017 00:46:50 -0400,
>> rickman wrote:
>>
>>> So my odometer is accurate and precise.
>>
>>I understand you because you're exactly the type of person that I had in
>>mind when I asked the question in the first place.
>>
>>> I don't know what you mean. I have checked my odometer against the markers
>>> on the highway as well as against my GPS (I think the highway markers are
>>> more accurate than the GPS). It is spot on with the current tires to 1% or
>>> better.
>>
>>Does your tripmeter have a decimal place and digits after that decimal
>>place?
>>

<snip>

>>Which is the entire point after all.
> The man is right You are wrong. You ASS U ME too much - and at the
>risk of insulting the few GOOD engineers on the list, you OBVIOUISLY
>are an "engineer", but not one I'd hire for a job. The job would come
>in WAY over budget, WAY late, and would need to be completely redone
>by techitians and technologists at great cost, or to save time and
>money, completely decommissioned and scrapped - starting over with
>someone who knew what thet were doing, and how to do it - engineer or
>not.

You know, this guy has a hard-on against "non-engineers" measuring their MPG.
Rickman above told him he uses his odometer, then he goes on about tripmeters.
I answered his main complaints in another post. That exchange went like this:

">+ Tripmeter accuracy is what in the average car over a 300-mile tank?

>+ Owners ability to "match" the previous level of fuel is what?

>+ Gas station pump reading accuracy is what?

I never used the tripmeter for MPG, because I never bothered testing them with mile
markers.
Matching gas level is trivial - and it only has to done at the beginning and end of the
trip.
Gas station pumps - I assume they are accurate, and can't control that anyway.
I'm confident that my measurements are accurate to within .1 MPG."

His response to me totally ignored those responses, and he posed the same questions again!
Then, for some reason, he stated talking about speedometers.
He's a troll.

[rickman]

Air resistance rises as the square of the speed. So faster is worse by more
than the linear proportion. I find I notice the difference when I drive
over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
to overcome air resistance. I don't know how tires impact the equation and
of course since all these speeds are in top gear the entire drive train is
turning 33% faster as well.

--

Rick C

[rickman]

Mad Roger wrote on 7/22/2017 7:42 PM:
> On Sat, 22 Jul 2017 00:46:50 -0400,
> rickman wrote:
>
>> So my odometer is accurate and precise.
>
> I understand you because you're exactly the type of person that I had in
> mind when I asked the question in the first place.
>
>> I don't know what you mean. I have checked my odometer against the markers
>> on the highway as well as against my GPS (I think the highway markers are
>> more accurate than the GPS). It is spot on with the current tires to 1% or
>> better.
>
> Does your tripmeter have a decimal place and digits after that decimal
> place?

I've never seen a trip odometer that didn't have tenths of a mile.

>> My speedometer is mechanical and so has a separate calibration factor.
>
> The speedometer example was only brought in to point out that the vain hope
> that averages result in better "accuracy" is patently false.

Only because averages don't impact the effect of limited accuracy, averaging
mitigates the effect of limited precision. But both precision and accuracy
impact the error in any one reading.

> Mom-and-pop type of people actually believe that a speedometer reads even
> close to accurately - and worse - some here propose the vain notion that
> the more readings they take, somehow (magically?) the more accurate the
> results will be.
>
> A speedometer that reads high isn't going to result in more accurate
> calculations even if you do a billion test runs.
>
>>> + A pumpmeter of 20.25 gallons is likely relatively accurate & precise
>>
>> Of course it is. States inspect them at some point.
>
> You don't seem to understand what accuracy and precision even mean.
> Haven't you taken even one science lab course?

I think you are missing something. What you replied do does not in any way
indicate a limited understanding of precision and accuracy. But affect each
measurement taken. An inspection measurement will require the combination
of accuracy and precision in that measurement be within some limit. What do
you expect them to do, take dozens of measurements? There are economic
considerations, especially since this is about economics anyway. It is to
prevent excess profits from being made by shortchanging the customers.

>>> + Matching fuel level in the tank isn't even close to accurate nor precise
>>
>> I don't agree. I let the pump click off and then continue to pump for a
>> number of more clicks until it cuts off immediately.
>
> I'm not at all surprised about your concept of the fuel-level estimation,
> and, in fact, you're exactly the mom-and-pop type person I was talking
> about when I opened the thread.
>
> I understand you.

Not sure what that means. What I am doing by repeatedly topping off is to
reach the point where the fuel in the filler neck is right at the nozzle so
it won't run anymore, but rather cuts off immediately. This results in a
very consistent fill level.

>> I always need to run
>> at least another fifteen miles before I am home so that is better part of a
>> gallon burned so I don't need to worry about the gas warming up and running
>> out of the tank. I believe this makes for very consistent fill ups.
>
> I'm sure you do believe that.

I think my consistent mileage measurements support my conclusions.

>> My MPG results pretty well show the consistency of my measures.
>
> I'm sure your MPG results support any theory you want them to support.
> I believe you.

You seem to be doubting my results. Are you suggesting I am fudging my data?

>> You know what happens when you assume... ;)
>
> You don't know how funny that statement was to me when I just read it now.
>
>> I see less than 19 or even 19.5 MPG.
>
> I bet you see that decimal place even though it's not in the tripmeter
> estimation nor in the filllevel estimation.

You seem obsessed with evaluating the resulting MPG measurement even though
you can't put numbers on the accuracy of the parameters that impact the MPG
errors. If you can't come up with numbers, your ideas are of no value. But
that doesn't mean the errors in my MPG measurements aren't as they appear to
be.

Actually, I do have numbers for the parameters. I know the mileage to a
fraction of a mile (even though a tenth mile out of 400 is far more accurate
than anything else involved) and I have no reason to doubt the pump giving
me 20.0 gal when it says 20.0 gal. I don't fill up at the same pump each
time so if some were off it would show up and I'd be able to identify which
pumps were inaccurate.

> You see, I understand you because you're the type of person I had in mind
> when I asked the question.
>
>> I think the consistency of my MPG readings show how well each of these can
>> be measured.
>
> I'm sure you do.

You keep saying this without indicating what you mean.

>> As you say, the pump is going to be dead on.
>
> Whoa! I never said the pump was "dead on" and anyone reading this thread
> who thinks I think the pump is "dead on" would have completely
> misunderstood everything else I said.
>
> All I said was that the inaccuracies and imprecisions in the pump reading
> are likely better than the otherwise astoundingly huge imprecision in the
> fuel-fill level estimation and in the lesser inaccuracy of the tripmeter
> estimation.

Lol! You see, I understand you because you're the type of person I had in
mind when I made that comment.

>> Other than scale
>> error which can be calibrated out the odometer will be very good.
>
> Define "very good" please.

Have done, 0.1 mile over 100 miles has been calibrated... actually, it was
much better than 0.1 mile since I can interpolate the analog dial. I don't
drive that stretch of road anymore, so I can't calibrate 100.0 miles
continuously anymore or I would.

>> Filling your tank can be good as well.
>
> I'm sure you believe that filling the tank is "accurate" since you
> calculate 19.5 miles per gallon and not something like 19.5 rounded up to
> 20 and then the error taken into account such that it's more likely
> anywhere between 19 and 21 mpg than it is 19.5 mpg.

Sorry, your sentence doesn't make sense to me. Can you construct it properly?

>> It's not like they design gas tanks to have air pockets.
>
> Actually, they do have air pockets.
> Those air pockets change in size based on temperature & pressure & fill
> level.
>
> Even the fuel changes in density based on those parameters.
>
>> You don't need to know any of this specifically.
>
> Of course I don't. 19.5 mpg is all I need to know.
> And if I change "something" which results in 19.7mpg, then of course, that
> something was the cause. I understand. I really do.
>
>> Why do you care which of the three has what specific degrees of accuracy and
>> precision?
>
> I care because when I do a calculation, my assumption is that 19.5mpg is
> actually something closer to 19 to 21 mpg than it is to 19.5.
>
> If the "change" I'm measuring is within that margin of error, then I can't
> say anything about what that "change" was.
>
> And, more importantly, neither can you.
> Which is the entire point after all.

If what you say is true, why is it I have only seen 21 mpg a very, very few
times in the 20 years I have been checking my mileage? If what you are
saying is true, I should see a much wider variation in measurements than I
see. As I have said, 95% of the time I get between 19.5 and 20.5 mpg or
within a 4% range (+-2%). It's actually even tighter than that. It's more
like 19.7 to 20.2 mpg but I can't say just how often.

--

Rick C

[pf...@aol.com]

KEEERIST!!!

Miles driven per gallon, kilometers driven per liter, furlongs realized per bale. Pretty basic. And within "R-E-A-L-I-S-T-I-C" tolerances, may be calculated using 2nd grade arithmetic.

However! There are various factors that will affect results:

Speed driven
Distance driven
Style of driving
Type of vehicle
Load on vehicle
Condition of vehicle
Terrain
Type of tires
Condition of tires
Tire inflation
Condition of bearings & CV joints (if any)
Condition of suspension

So, we are able to make a snapshot of any given trip. And an average of several trips, that will give us a practical expectation of consumption based on our style in our car in its present condition. Not to be confused with an actual and accurate description of consumption - as that not only can, but *W*I*L*L* change with any change in the above parameters, and likely several others not enumerated.

We are discussing CARS as they are used EVERY DAY. We are not discussing neurosurgery, rocket science, disease vectors nor anything else other than very broad-brush stuff.

Per Deacon Mushrat, 2,619 1/2 can dance on the head of a pin - so that is now settled science.

Yikes!

[root]

rickman <gnu...@gmail.com> wrote:
>
> Air resistance rises as the square of the speed. So faster is worse by more
> than the linear proportion. I find I notice the difference when I drive
> over 60. By 80 you are burning a *lot* more fuel than at 60, about 75% more
> to overcome air resistance. I don't know how tires impact the equation and
> of course since all these speeds are in top gear the entire drive train is
> turning 33% faster as well.
>

It is true that air resistance goes up a square of the speed, but
the power requirement, and the corresponding rate of fuel consumption,
goes up as the cube. Work=force*distance, Power=force*speed.

[Tekkie®]

Vic Smith posted for all of us...

>
> I never used the tripmeter for MPG, because I never bothered testing them with mile
> markers.
> Matching gas level is trivial - and it only has to done at the beginning and end of the
> trip.
> Gas station pumps - I assume they are accurate, and can't control that anyway.
> I'm confident that my measurements are accurate to within .1 MPG."
>
> His response to me totally ignored those responses, and he posed the same questions again!
> Then, for some reason, he stated talking about speedometers.
> He's a troll.
>

Exactly what I have been posting. This guy is the valve stem thread, bead
breaker, etc troll.

--
Tekkie

[pf...@aol.com]

Y'all are feeding the troll by responding at any level pasts 2nd grade arithmetic. Full Stop.

[rickman]

You are right that the horsepower requirement goes with the cube. But, that
doesn't impact the gas mileage. Since you are traveling faster you drive
for a shorter time, so that extra factor in power cancels out. No?

--

Rick C

[cl...@snyder.on.ca]

No, because the speed doubling takes only half the time, but 4 time
the power. Not necessarilly 4 times the fuel, because the engine may
be "on the cam" at the higher speed, running more efficiently.

An example of this was the 1975 Toyota Celica GT. With the 1975
gearing, it was actually most efficient at 80MPH in 5th, as long as
you didn't have to change speed or pass anyone. (I got 52MPG at just
over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
morning back in 1979-ish.

Didn't work on the 1976 model - same body (and engine) but different
gearing

[rickman]

What was the lowest speed you could use 5th gear in the 75 car?

--

Rick C

[cl...@snyder.on.ca]

Can't remember for sure, but it was a DOG at 60mph - requiresd a
downshift to get anywhere. I think hey geared the 75 GT the same as
the 4 speed. I know I was shocked by the mileage on that trip - going
out to Kingston to pit crew for Taisto Heinonnen, "The Flying Fynn"
and Tom Burgess on the Twin Lakes Rally. Crewsd for him on the Tall
Pines and the Blossom too.

I was offered his backup Celica Team car in 1980 when we finished
rallying in the navigational rallye series (After finishing 1st,
second and third in 3 years we were no longer elligible) and our R12
was not adequate to run competetively in the performance series but I
decided to quit while I was ahead, since I was getting married.

[rbowman]

On 07/24/2017 07:46 PM, cl...@snyder.on.ca wrote:
> An example of this was the 1975 Toyota Celica GT. With the 1975
> gearing, it was actually most efficient at 80MPH in 5th, as long as
> you didn't have to change speed or pass anyone. (I got 52MPG at just
> over 80mph from Waterloo to Kingston Ontario at 2am on a Sunday
> morning back in 1979-ish.

I had the misfortune to own a '71 Audi when the 55 mph national speed
limit went into effect. The German engineers thought 55 was a very brief
period on your way to cruising speed not a speed you'd try to drive.