[JayChicago]

Agree. Modern sender will never give accurate reading with an original gage.

But yours seems extreme.(needle always moving full scale) That’s not a lack-of-ground problem. Just the opposite, indicates possible short to ground. To test, remove the wire at sender and keep it from touching ground. Needle should then stay bottomed. If it does that, gauge and sender wire probably OK, problem is with the incompatible sender.