The relationship between air conditioner usage and gas usage is a tricky one to answer. It is heavily based on the situation you are in. Sometimes the air conditioner can waste your gas, but at other times it is the best way to save gas. Use of the air conditioner is going to cause the car to use more power, and thus use more gas, but sometimes it is more fuel efficient than not using it. The next two sections will explain when is the best time to use your air conditioner, and when to avoid it.
If you are driving at a speed of anywhere from 0 to 40 MPH, it is generally recommended you do not use your air conditioner. Instead, put your windows down. The drag on the car will be negligible, and you'll be able to keep cool. Using the air conditioner unnecessarily will use gas.
If you are driving at speeds greater than 40 MPH, it is more fuel efficient to roll up the windows and turn on the air conditioning. The rolled-down windows create a drag on the car and create a situation similar to driving into a head-wind. This will cause the car to use more gas than is used when you roll the windows up and keep cool with the air conditioner.
If at all possible, do your driving in the early morning or later at night, when the day is cooler. This will allow you to drive comfortably with your windows up and no air conditioning, which is the most fuel efficient way to drive a car.
