I have a Car and i have been driving my car without AC on, i always leave the windows down while driving, but i do see other people always driving with AC, i know of a friend who has never brought his car windows down since he bought his car. Though i have made a serious research about this topic, and i wanna share.
Well th thing is, the battery in the car has a rechargeable battery. The battery uses it's charge to turn on the engine in the car whenever you kick start your car. So once the car engine starts, it's self sustaining if there is fuel, while the engine is running, the alternator in the engine keeps recharging the battery using some of the power produced by the engine.
So now when you turn the AC on in the car it uses the energy supplied by the alternator. The AC won't be able to work until the engine is running, because the belt which only roles when the car engine is running is responsible for running the alternator and charging the battery.
With this knowledge i will come to the conclusion by saying running an air conditioner in an automobile decreased gas mileage by 5 to 10%. It has been recommended that is better driving with your car windows down when travelling on a long journey, but AC can be use when driving around the city. One important thing about keeping your car window down when travelling on a highway or on a high speed is because there might be a probability of something happening to your car and not knowing about thing when ll the windows are rolled down and on a high speed. I remember onetime i was on a journey and my car windows were all rolled up, at a time i observed smoke coming out from my car engine, i stopped opened up my engine and realised the engine belt is damaged because of the water was dried up. Thank God i was not on a very speed, it would have been a disaster. In essence, if my car windows were down, i would have noticed it in time and probably the car belt would not have damaged the way it did.