I recently changed my engine airfilter...the original was very dirty (after 25,000 miles of city/tollway driving). I've noticed about an average 2 mpg improvement in mileage since then. Is this possible? Before others respond that it must be related to driving habits, my route, speed and driving habits were essentially identical each day. I figure it must be coincidence, but I do not know how much a very dirty air filter will affect the mileage. Just curious.