First rule is to not use the Fuel gage to calculate MPG. The gage is notoriously inaccurate. The only true and accurate way to calculate MPG is to get an exact reading off the pump (down to the 10ths or 100ths of a gallon) when you fill up, and calculate that against the exact Odometer reading in 10ths of a mile. And even then, one tank of gas does not make an accurate Average MPG since there is not much to average until you have put about 5+ tanksfull and accurately calculated the mileage for each tankfulll, or combind.
...Rich