You own a sports car and you're worried about mileage? I haven't done the mileage equation since... ever.
Seriously... you have to fill up the tank... drive a few hundred miles... fill it up again... look at the diference in fill amounts... and do the math. Look up the equation on the internets.
The math? I'm not sure because I've never much oncerned myself with mileage in this car (smiles per mile...).
Further... 60 miles is not much of a trip to be calculating mileage. The fuel level guage starts going low quickly as you get to that last quarter tank and, if corectly I recall... when it's filled up. When the gauge is in the middle 50% slowly it moves... down. I don't know... I don't pay attention to it. I just try to keep my tank at least 1/4 full.
Look up the math equation... online... and try it. Let us know the real results... not the seat of the pants answer.