I changed my fuel filter on my Grand Am. Since then my Mileage is about 10 miles per gallon or less. I was getting 22 - 25 combined before. I have since changed the O2 sensor, done a injector cleaning, changed air filter, had a fuel inspection done on the car. I was told that if I took the car on a long highway drive it would reset something, so I did that and still my mileage sucks. What is going on?