Always wondered about this. If the computer in a car adjusts fuel ratio, when the air filter is dirty, and passing less air, and the computer is allowing less fuel, how does this effect mpg? I would think it would be like having less hp, and having to press harder on the pedal. On my 2002 Taxoma V-6, with 140,000 miles, and having only changed the air filter 3 times, the mpg. hasn't changed. (Scan Gauge) This is with the orig spark plugs also.