[BrianG]

With all the fuel sloshing around in the tank due to vehicle angle (including upside down) and bumps, it would be very difficult to make an accurate general purpose fuel level indicator. The best way I can think of is to make a fuel flow meter in the actual fuel line. You'd have to somehow program in the fuel tank capacity so it knows how full it is.

Easier to do away with fuel issues by going brushless. :)