Bad gas mileage after tune up

Hey guys, It's been a long time since i've posted on here. I have a question about my gt. It's a 97 with 91k, 5spd, I have a cia on it and thats it. First of all I just replaced my ac compressor. Then I took my car in for a tune up. New plugs and wires. After that my car started running must better than it had been in the past. I have had some issues with my idle surging and the engine running rough. The tune up seemed to rectify this but I still get horrible gas mileage. What could this be? Exhaust possibly? Thanks again.

CE

They did put in a new fuel filter I know that. I Don't know if they replaced the sensors though. Usually I get 300 Miles to the tank and Now its around 220. Maybe it's cause the AC has been at full blast down here in Phoenix. Luckily It's cooling off. Thanks for the help.