Q: Mpg going down

Hello. If the gas mileage has gotten worse since you replaced the air filter you will want to look into that first. It may be installed upside down or improperly. If the air filter is not the issue, then you should check the duct work and air filter housing. If there is a leak and the air pressure and vacuum levels are abnormal, the engine will be confused and run poorly. The spark plugs and ignition system can also have the same effect. When was the last time you had the spark plugs changed? You may want to do this just as normal maintenance, as it will not hurt the vehicle and can only improve the drivability. If the ignition system is working correctly, you will want to make sure your fuel filter and injectors are clean and working correctly.