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Could driving a vehicle at higher RPMs be more efficient?

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Dear Tom and Ray:

A friend and I are having a running debate that I was hoping you two would be able to settle. We each recently bought ourselves pre-owned vehicles, and we both are members of the dying breed of drivers who love to drive cars with manual transmissions. Both cars are in stellar shape, both have fewer than 30,000 miles and have their original clutches, both have spotless Carfax history reports, and both were given the thumbs-up by our mutual mechanic when he inspected them. The disagreement is about driving styles. My friend says that he gets better average fuel economy for his car when he drives at higher revs in a lower gear. I, on the other hand, believe that driving lower revs in a higher gear is more efficient. For example, he says that driving 60 mph in 4th gear at 3,000 rpm gets you better mileage than driving 60 mph in 5th gear at 2,500 rpm. What do you say?

-- Marie

RAY: While he's reading this first paragraph, Marie, we'll stall for time so you can double your wager. Because he's got his headlight firmly implanted in his taillight socket. He's nuts.

TOM: Completely. The higher the engine revs, the more fuel it uses. It's like walking. On the same terrain, the faster your legs move, the more calories you burn. And the same is true for your engine.

RAY: That's why, in chasing better fuel economy, carmakers have continually added more gears. In the 1950s and '60s, most cars had three-speed transmissions. The Ford Focus we drove a few weeks ago had a six-speed automatic. And the BMW X3 we drove last week had an eight-speed!

TOM: Why? Because the higher the gear, the fewer times the engine has to turn for each rotation of the car wheels. And the slower the engine turns, the less fuel it uses.

RAY: If your friend is really thickheaded, Marie, and refuses to concede, suggest that the two of you take a bike ride. Find a couple of 10-speed bikes (which won't be easy, because even bikes now have 21, 24 and 27 speeds). Then find a nice, flat road to simulate a highway.

TOM: Then set the rules: You get to use all 10 of your gears, but he can only go up to 3rd gear on his bike. Then race a mile. While you're pedaling easily and your bike is moving quickly, he'll be pumping his legs furiously and struggling like crazy to keep up with you.

RAY: At the end of the race, when he's exhausted, sweating profusely and doubled over trying to catch his breath, ask him how much "fuel" he used keeping up with you, and then collect on your bet, Marie.


Comments

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palmbeachpop

Tue, 2011-12-27 16:51

I'm told that truck drivers using their own fuel try to stick as close to 2200rpm as they can for best diesel efficiency. I suspect gasoline engines do about the same.


Lee_T

Tue, 2011-12-27 23:23

A "Car and Driver" editor once went even further - he said that driving in such a high gear that the gas pedal is floored yields the highest mileage because when the pedal is floored is when the engine has the highest volumetric efficiency. This was back in the days of carburetors so when the pedal is floored the throttle plate is completely open. I don't know if this applies to fuel injected engines.


Truck Driver

Thu, 2011-12-29 01:07

Big truck max out 2100 rpms - Many have trucks governed down to 1800 rpms. Every engine is a little different but figure around 1300 - 1400 rpm is were big trucks get best fuel.


Oldtimer2002

Thu, 2011-12-29 18:44

A gasoline internal combustion engine gets better efficiency when the vacuum pumping losses are reduced by opening the throttle valve wide open. Low rpm gives best fuel economy with a low overlap camshaft. Wide open efficiency with a carbuerator is difficult as the metering orifices usually open up mechanically with the throttle plate movement. Carburetors also have an accelorator pump that squirts more fuel with rapid throttle plate movement.


Wakebe

Sun, 2012-01-01 08:45

Sorry, while final answer may be right, didn't care for this explanation, maybe better without the cycling analogy. Still, there is an optimal RPM for efficiency whether its a piston engine, turbo-shaft, or human running or human on a bicycle. This seems to imply low RPM always more efficient, which is obviously not true, other inefficiencies will take over at lower cadence/rpm. In cycling optimal tends to be 90-100 RPM, running about 180 strides/min (with 2 strides per cycle its about equivalent to the 90-100 RPM). Same with car engines, there will be an optimal. Adding more speeds to a bicycle allows more variability to stay at/near optimal RPM depending on the conditions (wind, incline, terrain, etc).


Gearcat

Wed, 2012-01-04 03:21

Just because a transmission has six speeds doesn't mean it has higher or lower gear ratios. It's the same at each end with more gears in between. Okay, maybe they are slightly different at top gear, but not that much than a three speed.


CarFan

Thu, 2012-01-12 21:30

This is a bit more complicated than the bicycle analogy in that there are other factors involved as well. Torque/ gear ratios, engine size and the car's aerodynamic efficiency ratio. A Corvette can be 'tweaked' to get optimal efficiency driving at higher revs and thus higher highway speeds than a Camry. In addition low revs at high torque won't get you up a hill unless your driving a diesel. (Last time I rode my bike up a hill in a low gear I couldn't make it. because it required more torque/ drive energy, than I could provide.) Overall the idea is to keep the engine 'working' at the correct RPM it has been designed for. So his car may very well get better mileage at 3000 RPM and she might get better mileage at 2500 RPM (Although I think that 3K is a bit high for efficient operation.)


ShiftKicker

Sun, 2012-01-29 22:17

@palmbeachpop: Diesels get better fuel economy at 2200 because that is close to their torque peak. By this reasoning, gas engines would be most efficient at 4500 rpm or so. However, by this speed in top gear, wind resistance reduces efficiency so that gas engines would be more efficient at 2000-3000 rpm in the same gear.


ShiftKicker

Sun, 2012-01-29 22:20

Unless it's a turbocharged engine.


richalt

Sun, 2012-02-05 11:26

I have a car with an MPG readout and I definitely get the best mileage when I feather touch the accelerator and use the minimum amount of pressure. Get that engine down to the lowest, lowest RPM that keeps the car moving in an optimal speed range of 35 to 50 MPH. Slower than 35, then engine is producing more power than needed, even at idle. Faster than 50 the engine requires more accelerator pedal to maintain the speed. Optimum is the speed where you can sustain the MPH with a feather touch on the accelerator.


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