Ever seen this? Gas gauge reads correct up to half tank then goes to empty as you fill from half to full. As gas is used the gauge comes off empty until it reaches half then reads correct from half to empty. Problem showed up when I swapped in a rally cluster for the 150 mph speedo, worked properly before. I grounded the sending unit wire and it sweeps to full as it should.

The electrical connection on the rheostat is lost when the tank is over half full causing the guage to read empty. You need to pull the sending unit out of the tank and re-establish connection by carefully cleaning and probably bending the little arm that rides against the coiled spring looking thing (variable resistor)

I'll check that, thanks.

What Pac said, run it (close to) out of gas to avoid spillage & pull it. First tho I'd ohm the threaded stud to the round flat circular base & see what you get with it full and when you get it empty (just for a reference) but with the non linear readings you are showing it does have to come out

...and of course there is also a chance the coil windings on the rheostat are actually broken somewhere in the middle--thet would require replacement. You need to test it with an ohmeter when you get it out.