[Brian R.]

Don't judge the mileage by one fill-up. Total and average 4 or 5 fill-ups. What about on highway trips where you are only driving 55-60 mph for extended periods on flat terrain with the a/c off and the windows closed? This is where you get your best mileage and the best judge of what your car is capable of.

Are you filling the tank with the recommended octane gas as stated in the owner's manual? Lower octane gas can decrease your ignition advance to prevent knocking and lower your gas mileage. I believe your recommended octane is 91 or better for best engine performance.

Spark plugs?
Fuel injectors need cleaning?
How's the mileage with the a/c off as described above?
Don't idle for long periods.
Check parking brake adjustment.
Changed transmission fluid?
Alignment?
Check your intake duct for cracks and loose connections.
PCV valve