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Windows Open or Air Conditioning

Q: Does running the air conditioner make a car use more fuel? If so, would it be better to drive with the windows down when it is hot outside?

A: Running the air conditioner (AC) puts more of a load on the engine, resulting in higher fuel consumption. However, if you are running at highway speeds the AC actually consumes less fuel than if you drive with your windows down. When the windows are rolled down the aerodynamics of the car is less efficient, resulting in more drag. An increase in drag increases fuel consumption. Running the AC is more efficient than driving with the windows down at highway speeds.