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Air conditioning can reduce mileage significantly, by as much as 20%. In fact, your air conditioner can consume up to one gallon of gas per tank to cool the vehicle. But driving with your windows open can produce aerodynamic drag, which reduces fuel economy. What’s a driver to do? When driving at slower speeds (less than 40 mph), such as driving in urban areas, open windows are better. At higher speeds (over 40 mph), open windows use more fuel than the air conditioner, so close the windows and turn on the air conditioner. Another good idea is to take advantage of the “recycle inside air” feature. The air that is already cooled in the car is reused by the air conditioning system, instead of drawing hot air from the outside to be cooled.

Source: EcoDriving USA