Does 20mph in speed make a difference in fuel efficiency?

Dear Car Talk

Dear Car Talk | Dec 01, 1995

Dear Tom and Ray:

Please help settle a dispute with a colleague. She insists that there is a negligible difference in the amount of gas used going 70 mph vs. 50 mph. I say there is an optimum gas saving speed, and for most cars, that is around 50 mph. All else being equal, and you have one gallon of gas in your car, and it's twenty-something miles through the desert to the next gas station, how fast would you drive? Thanks for the info.

TOM: Gee, Jeff. With all due respect, I'd have to say that your colleague has her headlight in her taillight socket. And I mean that in the most respectful way.

RAY: The biggest issue in gas mileage at higher speeds is wind resistance. And resistance increases by the square of the speed. If you do the math, you can see that 50 (mph) squared is 2500, and 70 (mph) squared is 4900. So the resistance at 70 mph is almost DOUBLE what it is at 50 mph. And that makes a big difference in fuel economy. That's why aerodynamics became so important in the last fifteen years, and why all of our cars started to look like jelly beans.

TOM: And to answer your other question, the greatest gas savings takes place when the engine is turning its slowest. The slower the engine turns, the fewer explosions in the cylinders. And the fewer explosions in the cylinders, the less gas that gets used to make those explosions.

RAY: So to get the greatest possible gas mileage, you want the engine to turn as slowly as possible, while the car moves as quickly as possible. And you accomplish that by running at low engine speed in the highest possible gear.

TOM: So if I were in the desert, and had one gallon of fuel to get me twenty miles, I'd accelerate very gently and slowly until the car shifted into its highest gear (assuming it's an automatic transmission). And the minute the engine speed dropped as that last shift took place, I'd stop accelerating and hold the speed right there. On most cars, that shift to overdrive would happen somewhere between 35 and 50 miles per hour. And--provided you stayed in high gear and didn't downshift--that would be the optimum speed for maximum fuel efficiency.

RAY: And if it were really important to save gas, you'd want to keep the windows closed, turn the accessories off, keep the pop-up headlights down...and snap off that wind-resistant hood ornament just for good measure.

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