Drop in gas mileage

I'm going a little crazy here:banghead: . My 02 Grand Prix was getting between 28-30 MPG,then all of a sudden it started to drop little by little.It has 75000 miles on it so I decided it was time for a tune up ,so I replaced the plugs with autolite dubble platuim,replaced the wires at the same time,and no change in MPG.Then I went and had a wheel alginment and it didn't make any difference.I replaced the air filter and been adding fuel injector cleaner from time to time but no difference. Right now I'm getting 24.5 MPG and it really bites knowing I use to get 30.Anyone out there want to give their:2cents: on what I can do next. I'll try anything..................Thanks

It would be helpful to post the model GP and engine size.

It would be helpful to post the model GP and engine size.
I'm sorry wrote this in a hurry it's a 3800 GT.

You might want to have an exhaust back-pressure test done. GM has acknowledged that 01-02 GP may have been delivered with bad cats which can cause an exhaust restriction. Common symptoms are lack of power (especially under load or at high RPM), poor mileage, and random misfire codes. GM extended the converter warranty to 120k so you could be eligible for a free replacement.