Which is more fuel efficient? Drive with or without air-conditioner?

Many people thought that driving with the air-conditioner switched off and wind the windows down will actually save the car fuel.

I remember watching the Mythbuster solving this myth on Discovery Channel few years back.

They reached a conclusion that when driving at a speed that of less than 50 mph, it is more efficient to leave the windows down.

However, if the speed is over 50 mph, the drag is increased, and therefore, it is more efficient to leave the air conditioner on.

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