Well, not knowing too much about what you did when you repaired your car, I would have to say maybe the timing isn't set correctly? I don't know. I am assuming you just removed the radiator, pulley, timing cover and replaced the belt. The marks are supposed to be situated a certain way for correct timing. But on the other hand, the car seems to be running OK so maybe I am off. That is a huge drop in mpg's though.