I've always been curious about the accuracies of the fuel gauges of my harleys and also cars. There is no way a mechanical float operated fuel gauge can be accurate with an odd shaped, non symmetrical shaped tank without a great deal of more expensive technology. Look at the shape of the tank, nothing symmetrical about it.
I'm not sure about the later years with the mileage to go, but that feature is coming from the ECM with intelligence knowing how long the injectors are active, etc, past MPG, but the fuel gauge is actually a voltmeter. A set voltage usually 8 volts (to account for varying battery voltages) is fed to the float assembly which is a variable resistor that is operated by a float and rod assembly. Ground is the other end of the variable resistor. I have made these a bit more accurate (2000 Deuce) by bending the float rod. My FXR when full only reads 3/4, so if I bend the rod to read full it will be more accurate at the full end, but may be off at the low end.