Below I've pasted the total requirements (apart from deleting some paragraph cross references) from FAR-23, which is the safety code against which most light aircraft worldwide are certified, that affect fuel gauge accuracy. It wouldn't do to expect more than the aircraft was certified to do...
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23.963 (e) Each fuel quantity indicator must be adjusted, as specified ... to account for the unusable fuel supply...
23.1305(1) A fuel quantity indicator (must be supplied) for each fuel tank
23.1337(b) Fuel quantity indication. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used. In addition:
(1) Each fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined
23.1553 Fuel quantity indicator.
A red radial line must be marked on each indicator at the calibrated zero reading,
23.1551 (1) For fuel systems having no selector controls, the usable fuel capacity of the system must be indicated at the fuel quantity indicator