When the air conditioning is turned on, the compressor engages and puts a load on the engine, causing it to work harder. This increased effort consumes more fuel, resulting in reduced fuel mileage. The amount of fuel mileage decrease can vary depending on the vehicle, driving conditions, and the efficiency of the air conditioning system. On average, it is estimated that fuel mileage can decrease by about 5% to 20% with the air conditioning turned on.
