2008 Ford Fusion Gas mileage decline

I was getting between 30 and 32 miles to the gallon with my car. I put knew tires on and starting using puralator oil filters instead of fram. Now I am only getting 28 to 29 miles to the gallon. Wondering if either one of these changes would affect gas mileage at all that much or if there is something else. Also where can I find a repair manual for the fusion that would show me how to change plugs and wires.