Fuel gauge help

Well the sending unit could be stuck, the thing to do is wait until it's nearly MT and ohm the thing, then fill it 1/2 way and recheck, and then fill it and recheck.

I know that's lots of trouble, the alternative is yanking the sender/ tank.

I just found this, the old Miller gauge tester is alleged to be: (model C-3826)

L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)

NOTICE that the 1/2 way resistance is NOT "linear" that is, is not halfway between the high and low

You have four basic possibilities:

The gauge is screwed up (inaccurate)

The sender is screwed up (rusty, sticking, bent/ sinking float, etc)

The wiring is poor, poor connections, etc

The VR is irregular

If you buy some good heavy wirewound resistors the same value as the miller tester, you should be able to sub them back at the sender and check the gauge for accuracy