[Ronbo]

As I recall GM sending units are around 60 ohms but I think ford used a different value. Also the sensor can be 180 out from the gauge in some apps although fuel gauges are fairly universal. (there are always exceptions)

Big problem is the float arm needs to only have one bend max, two bends can cause a non-linear arc in the travel (changes length of the arm). Another factor is a non-square fuel tank. Idealy the resistor is located half way down in the tank.

The resistive element in the sender is wound nichrome wire so the resister is linear. (half travel is half resistance, a quarter is quarter, ect) There may be ceramic elements in use now but these didn't exist 20yrs ago so they have to match the wire elements properties.

Unless a guage is specifically adjustable at say 1/4 tank increments the only way to fix non-linear readings is to correct the sending unit.

In the case of a non-square tank the mfg has to calibrate the sensor to the tank. Typically by using bends in the arm or specific arm lengths and custom resistive elements.

Hopefully your never having to deal with custom stuff from a long gone mfg. This can be tough to get right.