Driving with the windows up and the air-conditioning on gives a car better gas mileage than rolling the windows down -- or so we've been told for years. But despite this advice, car owners continued to report that their miles per gallon (MPG) actually dropped when driving with the AC turned on. What's the best way to improve your car's MPG on hot summer days? Look at what the experts say and you'll understand how your car's gas mileage changes on warm-weather trips -- the answer may surprise you. Bad Gas Mileage Is a Drag Your car's drag is the amount of resistance it has to the air moving across it as you drive. Cars that have a shape that's long and lean (like a canoe) have less drag -- and usually get better gas mileage -- than a car or truck with a big, boxy shape. Driving with the windows down does change the amount of drag on a vehicle -- but only a little bit. A lot depends on your car's size, shape and speed. If, for example, you drive a ...