Mileage is one of those jpersonal things. Drive hard, and mileage dies.
Assuming you never jack rabbit the car (which would make you the only GT owner on the planet who drives this way), you can get about 300 miles per tank in mixed city-highway driving.
If you drive her hard and never leave an urban driving environment, with an AOD, you will get more like 200 miles per tank.
State of tune and condition have a lot to do with this. Get the car tuned properly, and the
tranny checked over, and make sure the brakes aren't scrubbing...
And you've done your job.
A simple CAI CAN cause you to run lean. This should be something to check during a thorough and expert tune up.