[The Deuce]

Analog gas gauges use floats in the gas tank. The mystery extra full may be your own doing. Do you squeeze every last drop into your tank at the station? It is possible to get at least one gallon into the tank and nozzle above where the float can go. It is also true that there is gas BELOW where the float can get to.

This may be the issue with your car. That or the float sticks a little bit. Either way, digital gauges guess too. There is nothing more accurate than doing the calulcations manually to figure out about how much gas you should have left.

I have had to do this from riding motorcycles and never being able to trust the fuel gauge. I'm also rarely more than a half gallon off what I expect to put into the car. But then I deal with numbers for a living, so I have some advantage.