Every engine needs three basic things to work: Spark, fuel, and oxygen. Some of the things that an engine does NOT need includes: dust, pollen, dirt, feathers and small rodents. A clean air filter allows oxygen flow while preventing the latter from being introduced into your engine. While a dirty air filter will keep all the dirt and small rodents out, it will also restrict the air flow and cost you precious fuel efficiency. In fact, changing your air filter is one of the easiest and most inexpensive things you can do to boost your fuel economy.