Start cheap, fuel filter, then I'd do the plugs & wires and see if that didnt help.
I'd also check under the hood and make sure thiers not any vacum lines loose/leaking,
replace PCV valve,
replace the air filter at every oil change(unless you use a washable one),
dont wait till the oil change light comes on(change around 3-4k miles).
If this is a GT you arent going to get realy good mpg anyway.
My SE gets great mileage doesnt matter if I run it hard or not.
My GT,
-it sucks!
Its been getting 18-20 doesnt matter city/highway.
(i suspect trans troubles)
The SE is 25+ more HP -
Might get it scanned if all that doesnt help, to see if your O2 sensors are working properly
hope that all helps.