Take it to the limit: On speed limits

I was babbling about the '70s energy crisis and the nationwide 55 mile-per-hour speed limit at work the other day, and found myself explaining to a group of younger people how you save gas if you drive slower. They had never heard such a thing!

Could you refresh my memory about why 55 is the magic number for saving fuel? They need to hear it in scientificese.

Ruth
Denton, Texas

Dearest Ruth,

As you well remember, 55 mph was decreed the national speed limit in 1973. It was lifted in rural areas in 1987, but stuck around as federal law until 1995.

Every single "Tips for Saving Gas" list still tells you to drive 55 mph, but very few tell you why. I'll tell you, though: physics. (Well, and politics -- a 35 mph interstate speed limit would not have met with favor.)

Here's the easy version of my physics lesson: Driving slower means less "drag," and thus less effort by the engine. Drag is aerodynamic resistance, basically.

You've felt it if you've ever walked with a banner in a parade or protest, or pushed against a heavy wind in a flapping coat. That's the same drag a car experiences as it pushes forward on the road -- it fights the air, it fights the friction of the road, it fights the urge to pull off and get a Big Mac. And instead of burning lunch to fight drag, it burns gas. The more drag, the more gas.

Now here's the scientificese. Physicists have an equation used to calculate drag on a moving object: D = Cd x r x V2/2 x A. The D is drag, and V is velocity. You see how V is squared? The other letters (which you have my permission to ignore) stay constant as your speed increases, but the V rises. Because it is squared, it has a large impact on D -- i.e., twice as fast is four times as much drag.

The faster you drive, the harder your engine has to work to maintain its speed, and the less efficiently it performs. Miles per gallon fall. (You can learn more from NASA.)

So is 55 the magic number? Well, by many estimates it's pretty darn close, though each vehicle has its own speed of maximum efficiency dependent on engines, car bodies, and driving conditions.

Gas mileage decreases rapidly at speeds above 60; boosting your highway speed from 55 to 75 can raise fuel consumption by as much as 20 percent. Driving at steady, reasonable speeds will save both gas and money, and keep you safe and happy.

Physically,
Umbra