Do Dirty Air Filters Affect Your Gas Mileage or Just the Engine Performance?
Nothing will reduce a car’s gas mileage and decrease an engine’s performance more than dirty air filters. First, for the engine to perform at its peak, it must have sufficient airflow. The air/fuel mixture is monitored by the vehicle's computer. So, if an engine does not get the proper air to fuel mixture, then the computer will signal that more fuel must be added. Therefore, more fuel is delivered until the proper mixture is attained to enable the vehicle to run. This situation in turn produces less gas mileage. Furthermore, an air-starved engine must work harder, thereby reducing the engines performance.
For more info, see How to Clean an Air Filter