To maximize gas mileage and minimize emissions, your air filter should be changed every six months or 15,000 miles. To optimize savings, change it yourself. You will need a flathead screwdriver and a new air filter. Park your car on a level surface and let it cool down. Open the hood and locate the air filter. In most cars it is located in a rectangular plastic case at the top or front of most engines. Flip open the medal clasps on the case. Pull the old filter out of the case. Hold the old filter up to the light. If you can see through the filter, it is still in good shape and can be put back in.  Slide in the clean air filter and close the clasps on the case, then close the car hood.