Will driving 55 mph save money on gas?

November 2005

Your Question

I know that we're supposed to drive the speed limit, and that they lowered the speed limit to 55 back in the '70s or something to save gas, but then they raised it to 65 again and we're having gas problems again. Since gas is expensive, can I use the '70s strategy and drive 55 and save money on gas?

Our Response

That's a good question, and fortunately there's an easy answer -- if you know how to think about the problem.

The amount of money you spend on gas and the speed at which you drive are linked. Look at it this way: if you drove at an infinite speed, you would use gas infinitely fast*, and would have an empty tank instantly. On the other hand, if your car is stopped, you are driving at the lowest possible speed, and you never use any gas at all. Graph this on a piece of paper and you will see that the faster you go the more gas you use, and the slower you go the less gas you use.

So, yes, you will save money on gas by driving 55, assuming that the alternative is driving 65 or faster. If the alternative is driving slower -- such as in a school zone -- then driving 55 won't save you a thing. In fact, it might get you arrested, which is generally expensive.

*Technically, according to the theory of relativity, you'd use gas infinitely fast by driving at the speed of light. A weird consequence of this is that by driving faster than the speed of light you'd actually gain gas, but that's a discussion for another time.

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