Is it more fuel-efficient to drive with your car windows down or with them up and the air conditioner on? Turns out, if you're on the highway going more than 55 mph, driving with your windows up is a much more fuel-efficient way to go. At high speeds, open windows create drag, which can reduce your car's efficiency as much as 20 percent. But at low speeds, when you're driving around town on surface roads, it's better to crack a window and leave the air conditioner off. There is less drag, and your car is producing less energy to operate, so feeling the cool breeze through the window is the greenest way to go then.