I check my mileage every tank and have seen everywhere from 80 MPG to 43 MPG (US gallons). As Dabrakeman notes, mileage is very much dependant on wind, speed and terrain. Also on fuel quality; E10 regular is not the same as E0 premium.
I live in dry, open country with not a lot of fuel stations, and even stations that don't carry more than one grade of fuel.
Used to push my GT to 140 miles between fill ups if need be, until one night in the middle of nowhere I ran out. Seems I'd been speeding a bit, into a headwind and found myself empty at 136 miles!
THAT wasn't funny. I now fill up at a max of 120 miles and carry a bottle of fuel.
Often the tank is nowhere near empty, but my fuel gauge is so wonky it reads empty by 100 miles. Very frustrating.