What is gas mileage dependent on?

Accipiter22

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Feb 11, 2005
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Ok, 3 scenarios.

You have a destination that is 10 miles away.

A. You travel 10 miles at 5 m.ph., arriving 2 hours later.

B. You travel 10 miles at 30 m.p.h. arriving 20 minutes later.

C. You travel 10 miles at 60 m.ph. arriving 10 minutes later.




Assuming you use the same car for all 3 trips, without any stops along the way, and an empty road so that traffic is no concern, which one gets you the best gas mileage?
 

senseamp

Lifer
Feb 5, 2006
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B. Fast enough to stay in top gear, yet not fast enough to where air resistance kicks in big time.

 

Zenmervolt

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Oct 22, 2000
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Depends on the gearing, but for most modern cars, you will get the best mileage at 30 mph. (Actually, you get the best mileage in modern cars between 35 and 45 mph, but that wasn't an option in your list.)

The largest factor in engine fuel use is RPM. You are better off, from a mileage standpoint (and ignoring the fact that many ECU's are programmed to run the engine rich at full throttle), to have the throttle wide open and the RPM's low than to have the RPM's high and the throttle only barely open. It's much more efficient to short-shift and let the car torque itself down the road.

ZV
 

Accipiter22

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Feb 11, 2005
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Originally posted by: Zenmervolt
Depends on the gearing, but for most modern cars, you will get the best mileage at 30 mph. (Actually, you get the best mileage in modern cars between 35 and 45 mph, but that wasn't an option in your list.)

The largest factor in engine fuel use is RPM. You are better off, from a mileage standpoint (and ignoring the fact that many ECU's are programmed to run the engine rich at full throttle), to have the throttle wide open and the RPM's low than to have the RPM's high and the throttle only barely open. It's much more efficient to short-shift and let the car torque itself down the road.

ZV

so basically be in the highest gear, with the lowest RPMs possible for that gear?
 

shocksyde

Diamond Member
Jun 16, 2001
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Originally posted by: Accipiter22
Originally posted by: Zenmervolt
Depends on the gearing, but for most modern cars, you will get the best mileage at 30 mph. (Actually, you get the best mileage in modern cars between 35 and 45 mph, but that wasn't an option in your list.)

The largest factor in engine fuel use is RPM. You are better off, from a mileage standpoint (and ignoring the fact that many ECU's are programmed to run the engine rich at full throttle), to have the throttle wide open and the RPM's low than to have the RPM's high and the throttle only barely open. It's much more efficient to short-shift and let the car torque itself down the road.

ZV

so basically be in the highest gear, with the lowest RPMs possible for that gear?

Yeah, think of it as riding a 10 bike. Your car gets much less "tired" when it's in a higher gear.
 

dustmann

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Jul 26, 2006
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You get the best gas mileage at the point where your motor is working at it's most efficient rpm and gear selection. This is different for every car, so there isn't really a set formula.
 

dustmann

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Jul 26, 2006
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Originally posted by: shocksyde

Yeah, think of it as riding a 10 bike. Your car gets much less "tired" when it's in a higher gear.

Actually, this isn't entirely true. As a cyclist, there is a certain cadence (rpm of your crank) that is more efficient to ride at. Normal people on bikes usually ride at ~60rpm, which uses more energy and muscle from your legs. 85+ is where you get the most efficient use of your lungs/heart.

This, of course, is different for everyone. I like to stay around 90, and Lance Armstrong won the Tour de France by keeping his cadence at ~115.

FWIW *shrug*
 

Jahee

Platinum Member
Sep 21, 2006
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Originally posted by: Zenmervolt
Depends on the gearing, but for most modern cars, you will get the best mileage at 30 mph. (Actually, you get the best mileage in modern cars between 35 and 45 mph, but that wasn't an option in your list.)

The largest factor in engine fuel use is RPM. You are better off, from a mileage standpoint (and ignoring the fact that many ECU's are programmed to run the engine rich at full throttle), to have the throttle wide open and the RPM's low than to have the RPM's high and the throttle only barely open. It's much more efficient to short-shift and let the car torque itself down the road.

ZV

Why is that? Just out of interest..
 

Gillbot

Lifer
Jan 11, 2001
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To keep the motor from running lean and causing damage plus a few other factors. Most cars make better power a tad rich.
 

Zenmervolt

Elite member
Oct 22, 2000
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Originally posted by: dustmann
You get the best gas mileage at the point where your motor is working at it's most efficient rpm and gear selection. This is different for every car, so there isn't really a set formula.

It depends on how you define "efficient". An engine is making the most power per unit volume of fuel consumed at its torque peak, which can be one definition of "efficient".

However, I get much better mileage at 45 (1,800 RPM in 5th) than I do at 85 mph (3,200 RPM in 5th, essentially the torque peak of the engine) because of the benefit of gearing.

And yes, for cars the most efficient is to be in the highest gear possible to be in without lugging the engine.

ZV
 

Zenmervolt

Elite member
Oct 22, 2000
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Originally posted by: Jahee
Originally posted by: Zenmervolt
Depends on the gearing, but for most modern cars, you will get the best mileage at 30 mph. (Actually, you get the best mileage in modern cars between 35 and 45 mph, but that wasn't an option in your list.)

The largest factor in engine fuel use is RPM. You are better off, from a mileage standpoint (and ignoring the fact that many ECU's are programmed to run the engine rich at full throttle), to have the throttle wide open and the RPM's low than to have the RPM's high and the throttle only barely open. It's much more efficient to short-shift and let the car torque itself down the road.

ZV

Why is that? Just out of interest..

There are all kinds of engineering answers about protecting the engine and such, but the main reason is pretty simple (or I'm just cynical, or both):

- Power is measured at wide-open throttle.
- Emissions are measured at part-throttle.

Programming the ECU to run rich at full throttle allows the manufacturer to rate the engine (truthfully) at a higher HP without sacrificing emissions evaluations.

ZV