Is There an Optimal Driving Speed that Saves Gas—and Money?

I don’t think anyone really enjoys filling up the gas tank. It feels like you are just standing around when you could be getting somewhere. If you get gas on your hands or shoes, you smell like it all day. And on top of all this, you actually have to pay for it. With gas prices over $4 a gallon (up to $6 in some states), we are all too painfully aware that gas isn’t cheap.

Of course, there are many things you can do to spend less at the pump. The US Department of Energy has some tips to help you save fuel, which include removing roof cargo and the extra stuff in your trunk, using cruise control, and even turning off your engine while waiting in the drive-through line. You could also reduce how much you drive by carpooling or working from home. Finally—and this is the important one—you could just drive slower. Every car gets better gas mileage at 50 miles per hour than at 70.

But that gives us an interesting problem to solve: What commuting speed saves you the most money?

Here is the dilemma: If you drive fast, it takes more gas, which costs more money. If you drive slower, it takes less gas, so you will save money. But you will also sacrifice time—time you could spend on the clock at work, earning money. There should be some optimal driving speed at which the total cost (gasoline plus missed work) is minimized. This minimum cost will depend on the fuel efficiency of your car and how much you earn per hour.

Fuel Efficiency and Speed

If you turn on your engine and just idle there, you are still using gasoline. (Electric vehicle drivers, this does not apply to you.) At a speed of zero miles per hour, your gas mileage will be 0 miles per gallon (mpg), since you didn’t actually go anywhere. Increasing the speed to 10 mph will also increase your fuel efficiency, since it can’t get any lower than 0 mpg. But it might not be the best gas mileage. Your car is still using gas just to run the engine (and the AC and your very loud radio), and you’re not traveling very far.

Once you get up to much higher speeds, you have some other factors that ruin your efficiency. One is all the air your car is now pushing against as it moves. Your engine has to work harder so that it can rotate your tires and keep moving the car forward despite this resistance. You can see the effect of air resistance if, while driving on level ground, you shift your car from drive to neutral and take your foot off the brake. The car will start to slow down due to this interaction with the air.

The magnitude of this backwards-pushing force, known as air drag, increases with increased speed. You can feel this drag if you stick your hand out of the window of a moving car. If the car is driving slowly, you barely feel that force. At highway speeds, it is quite significant.