First, I would try doing and BG fuel injection service and air induction service which usually give better gas mileage. Second, check the air filter, third, check tire pressure, fourth, check to see if you need a major tune-up ie plugs,wires,cap,rotor,
fifth, check to see if brakes are sticking in any way, sixth, check to see if overdrive is working properly(most trans shops with do diag. for cheap.) seventh, you can call the dealership with the vin # and tag # off rear and they can tell you whats in your rear.
I used to work at ford as a master tech for 5 years. One other thing I would check is the tires for uneven wear and check alignment on it because if the tires are off it can affect gas mileage.
Let me know
ross