Nature's Air Conditioner

Does it really save gas to roll down your windows instead of flipping on the AC?

By Brendan I. KoernerPosted Friday, Aug. 6, 2010, at 7:17 AM ET

It's summer, which means many drivers will be cranking up the AC or rolling down the windows to keep their cars from turning into mobile toaster ovens. Which method uses more gas? In a column first published in 2008 and reprinted below, the Green Lantern explained that it depends on how fast you're driving.

Like most people, I crank up my car's air conditioning on hot summer days. But my wife has recently been tsk-tsking me for this practice—she says the AC wastes too much gas, and that we should roll down the windows instead. But I've read that rolled-down windows also decrease fuel economy, since they increase drag. What's the most efficient way to cool ourselves while driving?

The rule of thumb is to keep the windows down while on city streets, then resort to air conditioning when you hit the highway. Every car has a speed at which rolled-down windows cause so much drag as to decrease fuel economy more than a switched-on AC. As you might expect, however, that milestone speed varies widely from car to car—and in some cases, it may be well north of posted speed limits.