Goalie, take a minute and review what was posted about how to calculate your mileage. You might have it right, but it really, truly sounds like you are calculating your mileage wrong and coming up with an incorrect (and way low) number.

Forget about the fuel tank capacity and what the fuel gauge needle says entirely - they have nothing to do with it, unfortunately. The tank gauge is not meant to be an gallon-accurate measurement (as the 11 gallon fill up your g/f put in at an eighth of a tank showed), and you cannot make any meaningful calculations with it.

There is only one accurate way to calculate your mileage:

1) The next time you fill up, zero out your trip odometer.
2) The next time you fuel up AFTER that, divide the number of miles showing on your trip odo by the number of gallons you put in the tank on that second fill-up.

If you had it right, please excuse my presumption. If this is not the way you've been calculating your mileage, try it again this way.