With gas prices on the rise, lots of sites are posting tips on getting better gas mileage. Bargaineering has tried to quantify some savings, Frugal Work at Home Mom has a really nice long list of suggestions from SavingAdvice and even automotive performance retailers are getting into the act.
All the lists seem to agree that a dirty air filter will lower your MPG. If you are thinking of changing your air filter, you should consider switching from disposable paper filters to a re-usable, washable air filter like those by K&N. The filtering is achieved through an oiled cotton gauze and as a result more air gets to your engine than with paper. This can lead to increased performance and less gas usage (and a little more zip in your car too).
That's dandy, but the extra bonus is that one high-performance K&N filter can last longer than your car. Clean and re-use it up to 25 times...and with typical street usage, clean it about every 50,000 miles and you will never have to buy another filter. In contrast, paper filters last about 10,000-15,000 miles and get tossed in a landfill. Ugh.
Of course a K&N filter costs more than a disposable paper filter. They are usually $45-$55 for the filter and about $10 for a cleaning kit (that you won't need until after about 50k miles) and paper filters usually run about $15. If you don't count the better mileage or treading a little more lightly on the earth you've still made your money back after about 40k miles of street driving.
I'm no expert so if you are interested, please do your own research. I'm just a happy customer and I'm comforted to know my air is flowing just fine and probably will be for another 6 years. Then I'll just have to clean it to get my MPG back up ;)