Your oil-change guy points out how disgustingly dirty your air filter is, so you fork over $15 for a new one. The good news is youll be upping your miles per gallon, as clogged filters decrease engine efficiency. But your old filter is probably going to a landfill. Next time, buy a reusable, cotton air filter that you can wash out when it gets dirty. K&N Engineering of Riverside, Calif., has been making such air filters (pictured) for more than 40 years, but others are on the market as well. They cost about $5 more. Buying one is the single most important thing one can do to greenify their ride, pledges the greenest auto mechanic I know, Somervilles Frank Marotta Jr.