Generally, when you use your car’s air conditioner you get one or two LESS miles per gallon than normal. Here’s a fix from Lauren Fix: once the inside of your car gets cooler than the outside temperature, switch on the “air recirculator” (usually indicated by two arrows going in a circle). This will recycle the chilled air from inside the car and blow it back onto you, so you stay cool without taxing the engine and you will use less fuel!