Gas mileage decreased after changing the air filter. What would cause this?

Your mileage will/should increase with the better air flow. What can cause this, Driving styles, colder weather, Etc. are you the only one driving it? It can be the other persons driving habits like jack rabbit starts, Going from the gas to the brake with no coasting. So it could be many things but the clean new air filter should not be one of them unless you knocked the wires off the air flow meter. But then the check engine light would come on depending on the year. What year,make %26amp; model, and how many miles are on what you have? That would help me to better answer your question.|||I would just wait for the cars computer to get used the change in air flow, your old filter must have bean pretty dirty. i would drive the car on the high way for a bit so the computer can adjust it self. If this does not do anything than get the computer scanned by a mechanic.|||Did you have an oil change at the same time??? Some oils will cause engines to lose MPG%26#039;s. Also. there is the possibilty that the design of your replacement filter is more restrictive than the dirty one you replaced. I hope you are using genuine Subaru parts! Also, locate and clean your idle air control valve. The VECI map under your hood should show you where is it This will cause waste of fuel, especially at idle and under acceleration. Good Luck!!|||10 days? You can%26#039;t average fuel economy out over 10 days. Even small changes in driving habits can have a big impact. It needs to be averaged out over months.|||Let%26#039;s look at it this way.





You had a dirty air filter letting less air in and the engine computer adjusted by adding less fuel to the mix.





Now, you have more air coming in and the computer adjusts by adding more fuel to compensate.





While I am not saying not to change dirty air filters, this is the trade off at times, assuming nothing else has changed..