I'm pretty decent at electricical. I've had this same issue on my wifes 67 jeep Gladiator. New sender ( 2 different styles) read correctly out of the tank when hooked up to the truck harness. It appears to read correctly when installed disconnected from the truck harness and checked with an ohmeter. I have an adjustable resitance box. When I connect that to the harness at the tank, the factory gauge the gauge reads correctly. I ran new wiring from the sender by passing the cluster connection directly to the gauge. The result? it reads 3/4 at full, still has plenty of fuel on E. Doesn't make any sence to me.
Doug