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Actually- this was an episode of MythBusters. It was actually proven that is more efficient to drive with the A/C on, windows up, rather than drive with the windows down. The reason why is air drag. With the windows down, the air coming into the cabin at speeds was enough to make the engine have to run a bit harder and use more fuel to compensate. With the windows up and AC on, the natural areodynamics of the vehicle took over, and the engine worked less and used less fuel.
What is more fuel efficient: Driving with the windows down or driving with the air conditioning on?