More fuel gauge trouble shooting

I have a 73 duster and the fuel gauge hasn't worked for a while, if I fill it, it goes to about 3/4 tank but will be to empty after 30-40 miles. the ground is good and the ohm reading at the tank and at the wiring harness at the are the same and seem accurate, right now its at 39.4 ohms at the wire that plugs in the cluster but no reading on the gauge. any ideas?