By the way, if you want a REALLY accurate fuel gauge, get a ‘fuel totalizer’ installed. A totalizer consists of a sensor which goes right in the fuel line, and measures the flow of liquid with a little turbine. The output is converted to a digital pulse, and used to iincrement a little counter mounted in the vehicle.
Such a system can show you total fuel burned, instantaneous fuel consumption, average consumption, etc. And it’s accurate to maybe a tenth of a gallon per tank.
The big problem is that it can’t measure fuel loss in other ways (evaporation, siphoning, etc). And you need to make sure to reset it when your gas tank is full, and you have to fill your tank right to the top every time.
That’s mainly why these aren’t used in cars (well, that and the cost). But they are used in aircraft all the time.