Could driving a vehicle at higher RPMs be more efficient?

Dear Car Talk

Dear Car Talk | Dec 27, 2011

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.

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