Is there a device for detecting miles/gallon?

80 views
Skip to first unread message

brettr

unread,
Apr 19, 2001, 11:28:07 PM4/19/01
to
I have a '90 Geo Metro. Is there a way for me to connect a digital device
that will read out, in real time, my miles/gallon?

Thanks,
brettr


El Jefino

unread,
Apr 20, 2001, 1:58:28 AM4/20/01
to
I suppose if your engine computer can spit out vehicle
speed and injector pulse witdth, and you knew the constant
for fuel pressure, a scan tool and laptop could figure it out...

Simpler mpg gauges might exist, but you need one that will factor the
FI fuel tank return line in....

If you're just trying to drive for economy, a vacuum gauge
is said to do wonders...

Jeff
brettr wrote in message ...

ALFRED B

unread,
Apr 20, 2001, 2:14:07 AM4/20/01
to
I thought JC Whitney sold one of those meters in their catalogs....


The.Central.Scru...@invalid.pobox.com

unread,
Apr 20, 2001, 9:53:10 AM4/20/01
to
On Thu, 19 Apr 2001 22:28:07 -0500, brettr <b...@centurytel.net> wrote:
>I have a '90 Geo Metro. Is there a way for me to connect a digital device
>that will read out, in real time, my miles/gallon?

They used to make them in the 70's and early 80's, but all of the companies
that did eventually went out of business.

You can find them in the same places that have add on cruise control
units. (nowhere).

Maybe J.C.Whitney still has them, but beware: 95% of their merchandise
is pure crap.

WTF do you want one anyway? Reset your trip odometer everytime you
fill the car and divide by the # of gallons needed to fill up.

TANJWarrior

unread,
Apr 20, 2001, 10:04:09 AM4/20/01
to
Thank you....finally the most intelligent answer in this NG!

I have a 1984 Porsche and the no matter what my MPG gauge says...I fill up
the tank and divide the mileage by the number of gallons.

I was just wondering. Were trying to see how many dollars you were saving
so you were willing to shell out $45 to $100 to get a MPG gauge. If you
really want one...check with your auto dealership first. Most MPG meters
aren't that good...except to get money out of your pocket.

George Jefferson

unread,
Apr 20, 2001, 10:14:32 AM4/20/01
to
:
:Simpler mpg gauges might exist, but you need one that will factor the

:FI fuel tank return line in....

devices that actually measure fuel flow are very very expensive.
(more than the value of a 90 metro).

There is really no practical way to do it accurately unless
it is built into the fuel injection computer.

Rick D.

unread,
Apr 20, 2001, 11:23:58 AM4/20/01
to
They still do. I think it's a glorified vacuum gauge.

http://www.jcwhitney.com/product.jhtml?CATID=4570

Rich

--


--------
All you will ever need to know to understand the true motives and inner
nature of all Leftists can be found by reading John 12:3-6 -Lowell Ponte
RickyRaccoon's Tree
http://rickyraccoon.tsx.org


"ALFRED B" <alfredB1...@prodigy.net> wrote in message
news:9bok4c$3a76$1...@newssvr05-en0.news.prodigy.com...

John Hudak

unread,
Apr 20, 2001, 12:56:30 PM4/20/01
to
Nice comment...WTF....If you would have read his post, he wanted
REALTIME mpg.
I haven't seen any decent ones. Most of them worked on airflow, mainly
because measuring fuel flow and that low of a flow rate is hard - i.e.
sensors are expensive. I made an accurate one long time ago (20 years)
that was microcomputer based, the design is long gone.....And I can't
remember the flow sensor I used either...sorry
JOhn

The.Central.Scru...@invalid.pobox.com

unread,
Apr 20, 2001, 1:01:25 PM4/20/01
to
On Fri, 20 Apr 2001 14:14:32 -0000, George Jefferson <gj...@one.net> wrote:
>:
>:Simpler mpg gauges might exist, but you need one that will factor the
>:FI fuel tank return line in....
>
>devices that actually measure fuel flow are very very expensive.
>(more than the value of a 90 metro).

$5? They don't have to be *that* accurate.

The.Central.Scru...@invalid.pobox.com

unread,
Apr 20, 2001, 1:07:32 PM4/20/01
to
On Fri, 20 Apr 2001 12:56:30 -0400, John Hudak <jhu...@sei.cmu.edu> wrote:
> Nice comment...WTF....If you would have read his post, he wanted
>REALTIME mpg.

As I said, nobody makes them anymore due to lack of demand.
As I said, your only chance is a 20 year old model sitting on a shelf
collecting dust at say a KMART or J.C.Whitney.

Neon John

unread,
Apr 20, 2001, 2:16:40 PM4/20/01
to

George Jefferson wrote:
>
> :
> :Simpler mpg gauges might exist, but you need one that will factor the
> :FI fuel tank return line in....
>
> devices that actually measure fuel flow are very very expensive.
> (more than the value of a 90 metro).

'bout $300 for an oval gear meter, the standard for measuring fuel
flow. If you have a $300 Geo handy, I'll take it off your hands.
Should make a good roller skate.

>
> There is really no practical way to do it accurately unless
> it is built into the fuel injection computer.

Gee, people in this group keep telling me I can't do things that
I've been doing for years. I sure am glad that I do thing and THEN
read this group!

Computing mileage on an EFI engine is as simple as totalizing the
injector ON time, totalizing distance traveled, multiply in a
conversion factor and display. Hardware needed is a Parallax BASIC
Stamp (about $50) and an LCD display board (about $90.) We use the
PWM input across an injector to grab the pulse widths. We use a bit
line to read the vehicle's VSS to get distance. We write a little
BASIC code to wrap it all together and viola! Mileage/trip meter.

There are several methods of calibrating the fuel injector flow.
The most accurate is to build my fuel injector flow bench and
actually calibrate the injectors.

(http://www.diy-efi.org/diy_efi/projects/inj_flow/inj_flow.html)

The next best way to do this is to hook up a pulse generator (or
even better a PC using my FIT software at the above URL) to the
injector, fire off the fuel pump, discharge 20,000 pulses or so and
catch the fuel in a graduated cylinder. The pulse width should be in
the 2 ms range. This must be done instead of just yanking the
injector open to measure the flow to account for the opening and
closing delays in the injector.

The crudest but still satisfactory method is to hook the Stamp up
with some code that will display the raw injector and VSS totals,
put an accurate amount of gas in the tank, drive until it's used up
(or fill the tank, drive, fill it again and record the amount), then
compute the volume per injector pulse by dividing the fuel used by
the total number of injector firings.

John

--
John De Armond
johngdDO...@bellsouth.net
http://personal.bellsouth.net/~johngd/
Cleveland, occupied TN

George Jefferson

unread,
Apr 20, 2001, 2:33:08 PM4/20/01
to

:'bout $300 for an oval gear meter, the standard for measuring fuel

:flow. If you have a $300 Geo handy, I'll take it off your hands.
:Should make a good roller skate.

but you do need two..

:Computing mileage on an EFI engine is as simple as totalizing the


:injector ON time, totalizing distance traveled, multiply in a
:conversion factor and display. Hardware needed is a Parallax BASIC
:Stamp (about $50) and an LCD display board (about $90.) We use the

:There are several methods of calibrating the fuel injector flow.

:The most accurate is to build my fuel injector flow bench and
:actually calibrate the injectors.


we have different notions of practical <g>

Robert Hancock

unread,
Apr 20, 2001, 6:53:47 PM4/20/01
to
Won't help - if you just put it in the fuel supply line, your fuel usage
reading will be way too high because that will include the fuel bled back to
the fuel tank by the fuel pressure regulator. You'd need to measure the fuel
injector pulse width and know the fuel pressure to know how much fuel was
going in..

--
Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from hanc...@nospamhome.com
Home Page: http://roberthancock.netfirms.com/


<The.Central.Scru...@invalid.pobox.com> wrote in message
news:slrn9e0qss.hma.The.Cent...@c1459607-a.arvada1.co.h
ome.com...

Allen Ashley

unread,
Apr 21, 2001, 5:21:49 AM4/21/01
to
The.Central.Scru...@invalid.pobox.com () writes:

>On Thu, 19 Apr 2001 22:28:07 -0500, brettr <b...@centurytel.net> wrote:
>>I have a '90 Geo Metro. Is there a way for me to connect a digital device
>>that will read out, in real time, my miles/gallon?

>They used to make them in the 70's and early 80's, but all of the companies
>that did eventually went out of business.

>You can find them in the same places that have add on cruise control
>units. (nowhere).


I have bought two of the add-on mpg computers at yard sales. One of
them had a Z8 processor which literally decomposed. The second,
and current unit, has a mechanical indicator of total fuel used,
and an electronic display of mpg. It is fun to watch the display
while you have nothing better to do on long trips.

It is possible to calibrate a simple vacuum gauge to read mpg. I have
posted the calculations and a computer program several years ago in this
newsgroup. The readout of the vacuum gauge is much more responsive to
road/throttle changes than the electronic unit. On long stretches of
I5 in California the vacuum gauge and electronic units yielded identical
results.

>Maybe J.C.Whitney still has them, but beware: 95% of their merchandise
>is pure crap.


I like Whitney. I like their add-on cruise controls and the conversion
kits to replace breaker-points. The tools they sell are the same kind
you get at Harbor Freight. I don't buy any of the vehicle cosmetics.

>WTF do you want one anyway? Reset your trip odometer everytime you
>fill the car and divide by the # of gallons needed to fill up.


The gadgets are fun for a while.

The.Central.Scru...@invalid.pobox.com

unread,
Apr 21, 2001, 5:59:55 AM4/21/01
to
On 21 Apr 2001 09:21:49 GMT, Allen Ashley <ash...@alumnae.caltech.edu> wrote:
>It is possible to calibrate a simple vacuum gauge to read mpg. I have
>posted the calculations and a computer program several years ago in this
>newsgroup. The readout of the vacuum gauge is much more responsive to
>road/throttle changes than the electronic unit. On long stretches of
>I5 in California the vacuum gauge and electronic units yielded identical
>results.

I dunno about that. There's very little correlation between manifold vacuum and
MPG. Gear ratios throw it to hell: you can get 7mpg at 30"M; or 30mpg at
10"M depending on the gear ratio.

Knowing where the gas pedal is is about as useful. Duh, uh, if the gas pedal
is floored, the gas mileage will drop. And that isn't a decent rule. You get
the best fuel economy if you accelerate fairly rapidly to your desired speed at
say 1/3-1/2 throttle and then cruise. The worse fuel economy is when in
anything but top gear.

- Philip -

unread,
Apr 21, 2001, 10:04:36 AM4/21/01
to
>
>I dunno about that. There's very little correlation between manifold vacuum
>and
>MPG.

With the advent of feedback fuel systems (excluding those morphadidte feedback
carburetor systems), I would agree with you. But back in the carburetor days,
the lower your manifold vacuum, the lower the fuel mileage went. This was
especially true with four barrel carbs.

> Gear ratios throw it to hell: you can get 7mpg at 30"M; or 30mpg at
>10"M depending on the gear ratio.

Again, I would temper your assertion with the presence of feedback fuel
injection.

>Knowing where the gas pedal is is about as useful.

If that is true, then of what purpose is any 'throttle position sensor' which
is universally present on 'feedback' vehicles?

>
> The worse fuel economy is when in anything but top gear.
>

This is true up to a point. With a 'feedback' vehicle, the air fuel ratio
won't go extraordinarily rich at low manifold vacuum. With a carburetor in
place, chugging the engine in top gear with the throttle more open makes the
manifold vacuum drops well into levels that trigger serious mixture enrichment.

~~Philip

"As for my past sins, either the Lord
has forgiven me or the statute of
limitations has run out"

The.Central.Scru...@invalid.pobox.com

unread,
Apr 21, 2001, 1:46:55 PM4/21/01
to
On 21 Apr 2001 14:04:36 GMT, - Philip - <chip...@aol.com> wrote:
>>
>>I dunno about that. There's very little correlation between manifold vacuum
>>and
>>MPG.
>
>With the advent of feedback fuel systems (excluding those morphadidte feedback
>carburetor systems), I would agree with you. But back in the carburetor days,
>the lower your manifold vacuum, the lower the fuel mileage went. This was
>especially true with four barrel carbs.

Only in a given gear ratio.

Made up example:
Sample 1: 5mpg, 10mph, second gear, 25"hg
Sample 2: 35mpg, 55mph, fifth gear, 10"hg

Fuck the vacuum display. The best economy, for any fuel delivery system, is
to briskly accelerate and get into top gear as quick as possible and then to
cruise.


>
>> Gear ratios throw it to hell: you can get 7mpg at 30"M; or 30mpg at
>>10"M depending on the gear ratio.
>
>Again, I would temper your assertion with the presence of feedback fuel
>injection.
>
>>Knowing where the gas pedal is is about as useful.
>
>If that is true, then of what purpose is any 'throttle position sensor' which
>is universally present on 'feedback' vehicles?

Perhaps engine computers aren't really concerned with having sensors for
a fuel economy display. Perhaps they're more concerned with drivability and
performance.

Robert Hancock

unread,
Apr 21, 2001, 9:51:56 PM4/21/01
to
Some of the things the ECM uses to determine how much fuel to inject are
engine speed, manifold vacuum, throttle position (including rate of
increase/decrease), engine speed, coolant temperature, air temperature..
Seems to me that you'd need to measure most of those things in order to get
a decent idea, unless you just had some device that measured the difference
in fuel flow from the supply line and the return line (it'd have to be
pretty accurate though, I think..) It would be trivial to include such a
function in the engine computer when they designed the car, since it already
knows how much fuel is going in, but I'm sure they didn't..

Another way would be to measure the exhaust, I think that exhaust gas
analyzers like they use for emissions tests can give a MPG reading. So just
dump one of those in your back seat, run the sniffer out the window and into
the tailpipe, and there you go :-)

--
Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from hanc...@nospamhome.com
Home Page: http://roberthancock.netfirms.com/


"brettr" <b...@centurytel.net> wrote in message
news:Z1OD6.37760$m8.4...@news.direcpc.com...

Allen Ashley

unread,
Apr 22, 2001, 10:18:18 AM4/22/01
to
The.Central.Scru...@invalid.pobox.com () writes:

Quite clearly it has to be assumed that such a calibration holds for a
single engine-wheel turns ratio. My other assumptions were a fixed
altitude, average air temperature, and midline stoichiometry. As I said,
I also have an electronic gauge that meters gas flow and drive shaft
turns to calculate mpg and it agreed with the vacuum gauge result
as nearly as I could read the latter. In contrast, the vacuum gauge
is nearly instanteously responsive which is the primary reason it
is hard to compare with the digital readout. On straight stretches
of I5 I was able to hold the speed constant for a long enough time
to verify the calibration.

As far as driving techniques which maximize economy, talk to the drivers
(or observers) for the Mobilgas economy run. One of the observers is a
friend of mine, and as I recall the rules are: avoid stopping, hold
a fixed throttle even uphill (damn the drivers behind), and accelerate
very slowly from a stop.

Mike Graham

unread,
Apr 22, 2001, 12:38:26 PM4/22/01
to

The.Central.Scru...@invalid.pobox.com wrote:

>Fuck the vacuum display. The best economy, for any fuel delivery system, is
>to briskly accelerate and get into top gear as quick as possible and then to
>cruise.

This seems a no-brainer on the surface, and yet I understand that engines
work most efficiently at their torque peak, which would tend to indicate
that running too high a gear could be counter-productive.

--
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Mike Graham | Steel-fitter and CWB-certified weldor
metalmangler at headwaters dot com | Raiser of horses
<webpage temporarily unavailable> | Tester of limits (esp. patience)
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=


George Jefferson

unread,
Apr 22, 2001, 8:37:10 PM4/22/01
to

: This seems a no-brainer on the surface, and yet I understand that engines

:work most efficiently at their torque peak, which would tend to indicate
:that running too high a gear could be counter-productive.

efficieny on a fuel per power basis is typically WOT near peak
power. This doesn't do alot of good to know in terms of getting good
MPG from an existing car, but it tells you you want
a small engine working hard rather than a big engine running
at 10% power.

~^Johnny^~

unread,
Apr 23, 2001, 12:31:50 PM4/23/01
to
On or about Sat, 21 Apr 2001 17:46:55 GMT, in rec.autos.tech,
The.Central.Scru...@invalid.pobox.com () did apparently
write:

>Made up example:
>Sample 1: 5mpg, 10mph, second gear, 25"hg
>Sample 2: 35mpg, 55mph, fifth gear, 10"hg

!

Made up is right!
25 "hg at sea level would require a lot of deceleration. At 10 mph,
you don't have much decelerating to do, unless you're descending one
helluva hill, in which case your mileage would be more like 135 mpg.


--
-john

~~~~~~~~
"Always listen to experts. They'll tell you what can't be done and why.
Then do it."
- Robert Heinlein
~~~~~~~~

Wesley Murphy

unread,
Apr 23, 2001, 2:09:53 PM4/23/01
to
To check mileage, I simply fill up the vehicle (quitting when the gas pump
kicks off) and reset the trip odometer. I'll then drive it until I fill up
again. I then divide the milege on the trip odometer by the number of
gallons that went into the tank to get the MPG. Then I reset the trip
odometer and start all over again. Of course, this doesn't provide on
demand info but it works for me.

I remember reading a while back a guy had his 300Z outfitted with a lot of
expensive equipment to track mpg by a company testing some new product. At
times, under hard acceleration, he was getting less than 1 mpg and at other
times, deceleration, he was getting over 100 mpg.

Wesley Murphy


The.Central.Scru...@invalid.pobox.com

unread,
Apr 23, 2001, 3:09:21 PM4/23/01
to

I saw something like that when I had a '85 nissen sentra, although not
quite that wild. It could get as low as 7-8mpg, as high as 70-90mpg and
if you were doing a long highway stretch at 60mph, it could get just over
50mpg.

However if the drive was less than 150 miles, just driving to/from the
highway, say 10 miles of city driving at start and finish, was enough to
knock the average into the high thirties.

Lev I.

unread,
Apr 24, 2001, 1:59:57 AM4/24/01
to
Hey I liked that last one.

If you're going to go filling/driving/filling then you just need a $.05 tool
to calculate milage -- a pen and paper.
It works in this wondrous way :
Fill up, record the milage.
Drive.
Fill up again, note how many gallons are filled, record new milage. Subtract
new - old milage, divide by # of gallons. Viola!

--Lev I.

Mike Graham

unread,
Apr 24, 2001, 6:07:45 AM4/24/01
to
Lev I. <con...@inimage.com> wrote:

>If you're going to go filling/driving/filling then you just need a $.05 tool
>to calculate milage -- a pen and paper.
>It works in this wondrous way :
>Fill up, record the milage.
>Drive.
>Fill up again, note how many gallons are filled, record new milage. Subtract
>new - old milage, divide by # of gallons. Viola!

I use a credit card for gas purchases, so I get a receipt that has the
amount of gas on it. All I do is record my odometer reading on the receipt.
I *don't* use the trip odometer, because then I can't use it for anything
else. Plus, the odometer is a lot easier to read than the trip odometer on
my car.

Reply all
Reply to author
Forward
0 new messages