Super Six fuel mileage

I don't think you're lying. I think your odometer runs (very) fast or your math is otherwise, ah, compromised. That mileage claim just is not realistic.
I do take in to acount roughly 12 extra mile that the odometer shows me with the bigger tires.

For all to know how I'm figuring the mileage.

X being the number of traveled miles

X/number of gallons put in the car to full.

Say I travel 300mi.

300mi/15ga=20mpg