Q: Is it more fuel efficient to...

Q: Is it more fuel efficient to drive a car with the windows down for cooling or with the air conditioner on?

A: For speeds below 40 mph, it is slightly more efficient to have the windows open. But because of air drag, it becomes increasingly more efficient to use the air conditioner as speeds rise above 40 mph. In cars, the amount of engine power needed to overcome the air drag created by leaving the windows open increases with the cube of the vehicle’s speed--twice the speed requires eight times the power. It takes five horsepower to overcome the resistance at 40 mph, according to Science and Technology Desk Reference, but 18 horsepower at 60 mph and 40 horsepower at 80 mph.