It is also true to say that guages function on very low currents and poor connections may have some bearing on results. I have been told that in order to get correct fuel gauge readings an London Taxis they had to run an earth wire direct from the sender to the battery.
Sadly true especially of the lever float type where the arm tracks a slider over bare copper resistor coil. Because it has to move, the sliders electrical connection to ground (tank) is via the float arm pivot. Slightest corrosion on this can give higher resistance which gives higher or lower gauge reading depending on design.
Not sure if it would work on a car but a previous boats water and diesel tanks had long vertical sensors with a series of reed switches along their length actuated by a concentric float with magnet. This brought various resistors into circuit to allow corresponding gauge indications. There was also a make/break reed switch for low warning light. Reliable as no moving parts on electrical circuit.
