Dirty air filters can decrease fuel economy and horsepower by preventing the proper amount of air from entering your engine for combustion. Various sources claim that a very dirty, clogged air filter can reduce fuel economy by as much as 10% in the very worst cases. This would occur from an air fuel ratio where excess gas exists do to the limited incoming air from the clogged filter. This unique situation would result in not only a decrease in fuel economy but also a loss in power. In most cases however a few percent decrease in gas mileage is more likely.
1. When to change air filter
Many car manufacturers support replacing air filters every 15,000 miles. You may also change them earlier if they appear visibly dirty. Our general rule of thumb is to change them whichever comes first. However, some aftermarket companies such as K&N claim their high flow filters do not need to be cleaned for 50,000 miles. These are the non-paper types of cotton gauze filters that are typically cleaned and re-oiled instead of discarded after use.
2. What to Buy
When replacing an air filter any drop in replacement paper filter matched for your car’s year and model will do. If you are also interested in increasing horsepower and torque however you may want to consider an aftermarket high flow air filter along with air intake tubing, commonly referred to as an “air intake system”.
We do not have any documented evidence of increasing fuel economy after replacing a dirty air filter as we have not performed this experiment yet. However, aftermarket high flow air intake systems certainly do increase horsepower from our experience. Whether or not they increase fuel economy is up for debate.