When using the air conditioner, a vehicle uses more gas versus driving without the AC on. The difference between driving without the AC can result in a few extra miles per gallon.
Driving with the AC on requires more energy to be used by the car, resulting in the engine working harder to maintain the speed and increasing the need for gas. Cars without AC units get higher gas mileage since there is no added weight in the vehicle.
Rolling down the windows is often seen as the best option to avoid gas consumption, yet studies have found that driving with the windows down can result in just as much gas consumption when compared to using the AC. Only when going a slow distance will rolling the windows down save gas.