Originally Posted by mariusvt
It should help, also keeping your foot out of the skinny pedal so much would help too. You're likely using more gas because you are using the go pedal more to enjoy the gains from the intake & tune.
Originally Posted by mapitts
You bought a GT for MPG?
...and definitely THIS ^, too.
You're likely on it a lot more. Typically, if you drive the same as you did with the stock tune, you should see little change in fuel mileage.
It takes quite a few tanks of fuel and the awareness of whether or not there's anything from tank to tank that could be skewing the data to be able to discern any REAL gains or loss in fuel mileage. That includes having a ton of pre-tune data to back that up. If you're not hand calculating and logging that at each fill-up, you actually have no idea what your average fuel mileage is. The on-board is worthless for that.