Forgive my ignorance, but is that the 4 cylinder model? Is the GT a one engine model? It's hard to relate without more info about engine size and I don't keep that info in my Head Hard Disk.
If it's a V8, that's GREAT mileage .. even a 6 it is as my V6 gets only 19mpg on it's 330 hp. (19 mph as calculated readout by the car since bought)
Also, if it's an "instant" calculation I can make my car read over 30mpg .. downhill with clutch in. eheh A value calculated since you bought it is the only one that means anything as far as these new-fangled readouts go.