Checking miles per tank is not and cannot be accurate. Each gauge is slightly different an they do not even read incremently, ever notice it goes down faster under the 1/2 mark?
The best way to check mileage is to completely fill up, drive a majority of the tank away and fill it completely up again. I would suggest using the exact same gas pump both times.
See how many gallons it takes and then see how many miles you went.
Miles per gallon is going to give you a much better idea of where you are at, but that is not even exactly accurate.
Testing it this way, you should see 24+ mpg with highway, unboosted driving.
Driving around town and boosting a lot is going to bring that down.
I get about 28 mpg in Hyde on the higway, but I get about 7-to-11 mpg when racing.