For many years, drivers believed that replacing their air filter would help improve their fuel economy. A recent study has shown that's not actually the case with fuel-injected engines, according to the Department of Energy. However, it can indirectly affect the amount of gas that you use.

A clean air filter won't turn a gas guzzler into an eco-friendly hybrid by any means. However, it has been proven to improve the acceleration time of a vehicle by 6 to 11 percent. This means that you don't need as much gas to reach top speed, so you can ease off the accelerator a little bit. While not directly contributing to a boost in MPGs, drivers who keep this in mind can probably end up using less gas.

Plus, a clean air filter still has positive effects on a carbureted engine. On these vehicles, MPGs are actually affected - by anywhere from 2 to 6 percent on average. If the filter was really clogged, to the point of affecting drivability, it could actually improve fuel economy by 14 percent.

Thus it makes sense to have a clean filter and ensure you're getting the most out of your vehicle. So when's the best time to replace an air filter? In short, "when it's dirty." However, don't overreact if you open up your filter and see some dirt on there. That's what the filter is supposed to do - let air in while keeping gunk out. It's actually a good thing to have some large particles on the air filter - these will block smaller ones from entering your engine. The problems start when the filter gets so clogged that it blocks air as well - the one thing it's supposed to let in.

Once the filter is removed, a mechanic will typically clean out the housing with compressed air. You can do this at home with a vacuum cleaner. Then it's a simple matter of taking your new filter and placing it where the old one went.

If this is your first time cleaning your air filter, you might now know when it will need changing again. It all depends on how quickly it gets dirty and clogged. A good rule of thumb, however, is to check every 10,000 miles, so make a note of the mileage on your car when you're done so you know when to check again.