Re: 7 More Fuel-Sipping Myths Debunked: Mechanic's Diary

[mg]

On May 25, 1:54 pm, Sordo <sordo @ privacy.net > wrote:
> 7 More Fuel-Sipping Myths Debunked: Mechanic's Diary
>
> It's $4/gallon season, and the snake-oil salesmen are out in force.
> Forget the fill-up tricks, premium-gas ruses and miracle-gadget cons:
> PM's no-bull senior automotive editor is here to remind you that the
> truth is out there.
>
> By Mike Allen
> Published on: May 22, 2008http://www.popularmechanics.com
>
> For nearly 20 years, I've been giving all sorts of advice in my Saturday
> Mechanic column on how to improve fuel economy. Much of it has been
> fairly routine, like checking your tire pressure or tuning up your
> engine. And some has been plain-old common sense, like not flooring the
> gas pedal. Most folks forget about that last one. And while a new crop
> of so-called ecomodders like our guest MPG geek Benjamin Jones do a nice
> job of developing a new trick here and there, the real fear when gas
> prices get this high may not be so much what you pay at the gas station
> as the snake oil everyone's trying to sell outside of it.
>
> As legitimate analysts and agencies continue to threaten that the gallon
> can well exceed four bucks, other seemingly reputable sources are once
> again firing their silver bullets from the sidelines of the automotive
> industry. And once again, a lot of what I'm hearing is based on urban
> legends and a general lack of understanding about how cars actually
> work. What's worse, these people have agendas. And those agendas include
> removing money from your pocket by selling you stuff that's supposed to
> improve your fuel economy. Let's poke some holes in the new hype.
>
> Myth No. 1: Cold-Weather Fill-er-Ups
> The Lie: "Make sure to top off your tank first thing in the morning when
> the fuel is cool for more density and energy!"
>
> The Truth: Yes, cooler fuel is denser and, technically, will have more
> energy per gallon. Not that your car cares, because it automatically
> corrects the mixture ratio so the proportion of air-to-fuel is
> chemically correct. And it doesn't matter whether fuel is below freezing
> or so warm that it almost boils in the fuel rail—your car will
> compensate. So why should we care how dense the fuel is when it comes
> out of the pump? We don't buy fuel by its energy content—we buy it by
> volume. In theory, then, we could get more for our money if the fuel is
> cool.
>
> So the myth is partly right, but there's virtually no difference in the
> temperature of fuel stored in underground tanks as the day progresses.
> The tanks are buried deep enough to keep the temperature constant,
> although there are seasonal changes. What's more, this myth ignores the
> fact that most modern fuel pumps are temperature-compensated to equalize
> out any density difference.
>
> Myth No. 2: The Clean Air-Filter Swap
> The Lie: "If there isn't enough air, a dirty cleaner will make your
> engine burn more fuel!"
>
> The Truth: Back when cars had carburetors, changing your air filter
> could have helped. Today, the on-board computer injects exactly the
> amount of gas your car needs. A small restriction in the air flow won't
> make the engine run too rich. The pleated-paper air filters used today
> are very large, and can hold a lot more dirt than you might think before
> they start to choke the engine. An oily filter—the result of blow-by
> fumes from the crankcase—can make a filter plug up pretty fast. Modern
> crankcase ventilation systems are far less prone to oil them up than
> they used to be.
>
> Still, that filter probably won't affect mileage until it's so plugged
> that it makes the Check Engine light come on. But if you have an older
> car or truck, or any equipment that still uses a carburetor, it is
> important to keep an eye on the air filter, to make sure its
> free-flowing.
>
> Myth No. 3: Going Premium
> The Lie: "More expensive fuel offer better fuel economy, because it's
> higher quality!"
>
> The Truth: The media has confused this issue terribly. Post-1996
> model-year cars virtually all have knock sensors. If the octane rating
> is too low, the computer will roll back the ignition timing a few
> degrees to compensate. This will reduce peak engine power, and also
> increase fuel consumption. So, in some sense, this myth gets it right—if
> your car is supposed to run premium. One of our long-term test cars in
> the PM fleet delivers an impressive 25-percent better fuel economy
> running on premium than it does on regular, although our test was hardly
> done under rigorous conditions. I'll leave the math for an exercise, but
> I calculate the price differential between 87- and 91-octane fuel at
> more like 6 percent—at least in my neighborhood. Your mileage may vary,
> but it's worth trying to see what happens in your car. If you top
> 6-percent better fuel mileage on premium, it may save you money to run
> it.
>
> Of course, older cars, built in the days before knock sensors, may be
> damaged by running too poor a grade of fuel. Spark knock can actually
> burn holes in pistons, so burn substandard fuel in
> non-knock-sensor-equipped cars at your peril.
>
> But wait, there's more! Is your car supposed to run on Regular? There
> are more BTUs (energy) in regular than in higher grades. You may very
> well get better miles-per-gallon from regular.
>
> Myth No. 4: Tire Inflation
> The Lie: "Hey, hybrid owners and hypermilers: Overinflating your tires
> will reduce rolling friction and increase economy!"
>
> The Truth: Underinflated tires do have more rolling friction and waste
> gas. They also have less traction and stability because the low
> pressures don't stabilize the tire carcass, letting it squirm around.
> There is also a lot more internal friction in an underinflated tire, and
> that wastes gas. Overinflated tires also have less traction because the
> tire contact patch gets proportionally smaller as the pressure goes up.
> I say keep the pressures where the car manufacturer suggests. As always,
> check the owner's manual and follow tire-pressure recommendations.
>
> Myth No. 5: Cutting the A/C
> The Lie: "Leave off your air conditioner and open your windows, and
> you'll get more MPGs!" (or: "Using your A/C on hot days actually uses
> less fuel, because your aerodynamics are better with the windows up!")
>
> The Truth: The aerodynamic drag of any vehicle is proportional to the
> square of its speed. But the fuel consumption is proportional to the
> cube of its speed. At low speeds around town, the 5 to 8 hp necessary to
> run the air conditioner will decrease fuel economy measurably, while the
> small aero drag from opening the windows is negligible. Pull out onto
> the freeway and increase speed for 35 to 70, and the numbers reverse.
> Well, actually, the A/C consumes exactly the same amount of power—but
> the extra fuel required to run with the windows open goes up by a factor
> of eight.
>
> Either way, when you come back to the car after parking in the sun, open
> all the windows for a couple of minutes until the sauna inside cools
> off. That way the A/C won't be working so hard to cool off things
> initially. After your car's interior is closer to equilibrium with the
> outdoors, it won't take as much power—or gas—to keep cool.
>
> Some hybrids have very efficient, electrically-powered A/C compressors,
> and usually have highly optimized aerodynamics. So their low power
> consumption and higher aero losses, percentage-wise, make the break-even
> point at a lower speed.
>
> Myth No. 6: The Gadget Con
> The Lie: "Just add this magnet to your fuel line, or this high-tech
> whirligig to your intake, or pour this magic dust into your tank, and
> enjoy a 30 to three hundred percent bump in fuel economy!"
>
> The Truth: Rubbish, I say. I've tested dozens of these things. The
> Environmental Protection Agency has tested hundreds. Trust me, if a $10
> gadget could improve fuel economy by any significant amount, the car
> company that used it would have a crushing advantage in the world
> marketplace. And they don't, because that stuff simply doesn't work. You
> can't run your car on water. Most of these
>
> Myth No. 7: Short-Stop Idling
> The Lie: "Keep the engine running while you pick up the mail and a cup
> of java, and you're save gas fast!"
>
> The Truth: Your car generates negative miles per gallon if it's running
> and not moving forward. It's burning fuel— enough, in fact, to
> substantially lower your economy even when idling at traffic lights or
> in slow traffic. On the other hand, once the engine is warmed up, it
> uses no fuel whatsoever to shut down or restart when you need it. None.
> That's why an integral part of the fuel-saving strategy of any hybrid is
> to turn off the engine completely when the car isn't moving, or moving
> slowly. Don't leave your car idle, even for 30 seconds.

I think I used to get better gas mileage in my 1978 Chev 4x4 pickup
when I drafted Semis.