My best guess... There is already a aftermarket sender in there. Those are linear in resistance so they don't function like OEM which was non linear. In every case, The sender is the first look simply because is a lot easier to R&R. Then R&R it again to bend on the float arm, whatever in attempt to make it more accurate at the gauge. If all else fails, R&R the inst' panel to have the gauge rebuilt could cure it or at least help to some degree. There is a vendor out there who rebuilds the OEM senders but you would have to first have that part.
