The air conditioner puts extra load on the engine, forcing more fuel to be used. On average, a car operating with the air conditioning engaged uses about 20% more fuel. If it’s just too hot to bear without A/C, try to keep it set at around 75 degrees, and don’t blast the air when first getting into your car on a hot day. Open the windows initially, then close up the windows and set the air at 75. You’ll be surprised at how quickly your car will cool down once in motion.