guage or sender ?

[hfd410]

My 71 reads 1/4 tank when full. New 0-90 sender in under bed tank, all new wire. I ran a ground from the sender to the - side of the battery, no change. I also unplugged the brown sender wire from the panel and it read full. Grounded the tab and it read empty. Same thing in the back at the sender. Unhooked, it read full. With the sender wire grounded, it read empty. During the test in the rear I used the ground screw that the sender is grounded to so Im certain the ground is good. I can get to the tank easily and pull the sender with my lift...any advice appreciated.