Fuel mileage in vehicles directly corresponds to the amount of "load" placed on an engine. The more accessories that are driven by an engine's revolutions and power, the more fuel economy decreases. Air conditioning is a subsystem that produces a significant load on an engine, which directly affects fuel economy. This increase in engine load has been acknowledged and studied by agencies such as the California Air Resources Board.
Using the air conditioner in a vehicle may reduce fuel mileage in certain circumstances. This is not universally true, however. An air conditioning condenser is an accessory driven by a vehicle's main drive belt. The condenser is the primary mechanical element of an air-conditioning system, and is the most power-hungry element. The speed of the main drive belt directly corresponds with the engine's revolutions per minute (rpm). As engine rpm increases, the air-conditioning condenser can more easily generate cool air.
Higher engine rpm translates to an increase in efficiency in a vehicle's air conditioning system. This is because when a vehicle operates at low speeds (and lower rpm), the air conditioning condenser demands a greater percentage of the energy generated by the engine. EPA studies on air conditioning operation support this conclusion. At highway speeds, an engine is generating higher rpm and naturally generates more power. The air conditioning condenser's draw on engine power at higher rpm is largely the same as it would be at lower rpm. Thus, at higher rpm, the drain on engine power does not affect fuel economy as much as it would at lower rpm. This is because an engine is not losing as much power and efficiency at higher rpm because it is generating more overall power for the condenser to draw from.
Air conditioning's effects on fuel economy are directly attributable to the amount of "load" placed on a given engine. Rpm and speed are not the only factors that directly affect fuel economy when using air conditioning. If an engine is driving and powering more accessories, engine load will increase. The fewer systems demanding engine power to operate, the lower engine loading will be. EPA's study on mobile air conditioning systems finds a direct correlation between engine size and the amount of "load" placed on an engine while operating air conditioning. Bigger engines making more horsepower are not affected by the load (or demand) from running air conditioning, whereas smaller engines will experience greater load from running air conditioning. Thus, vehicles with more powerful engines might not experience the same decrease in fuel mileage as less powerful engines. This is due to air conditioning loading.
The amount of time and mileage of each trip should always be a consideration. Shorter trips might not require using air conditioning, or simply a slower fan speed. Consider using a warmer temperature setting or lower fan speed on shorter trips to minimize the impact of air conditioning on fuel economy.
On hotter days, briefly opening windows to flush out warm air early in a trip will offset the amount of air conditioning required to keep a car cooler. Keeping windows cracked while parked will keep the interior temperature cooler, thus requiring less air conditioning when operating a vehicle. These are often-overlooked means of supplementing a vehicle's air conditioning system to maximize fuel economy.
