As summer approaches, how do you best keep cool in your car? Is it better to crank the air conditioner or lower the windows to save energy? This question isn’t as simple as some people make it out to be.
Yes, using the air conditioner lowers your vehicle's fuel economy because it uses power from the engine (unlike the heater, which simply borrows engine heat). However, at high speeds, lowering the windows creates drag that requires the engine to work harder, also lowering your mpg.
The key is aerodynamics. If you have a sleek, wind-slashing vehicle like a Chevy Corvette, it will get better highway mileage with the windows up and the A/C cranked. On the other hand, if you're doing 65 mph in a Chevy Tahoe, aerodynamics aren't going to make a bit of difference anyway.
Essentially, though, the lower the speed, the more sense it makes to forgo A/C. Once you hit the highway, though, the less drag the better.