Problem could also be the 12 volt to 5 volt regulator on the backside of your dash. If it's wiggy and fuel gauge is not getting proper voltage , your gauge will be off.
I replaced my old mechanical one in my 71 Cuda's rally dash with a new solid state unit.
Thanks for letting me join.
I have owned my 1971 ‘Cuda 340 Winchester Gray since September of 1973. My cars build date is 9/18/70
Currently making him nice again. White high back bench seat and auto on column. BS23H1B1