Fuel sending units read WRONG time after time!

If it reads correctly at empty and full travel outside of the tank, then it must not have the correct travel for the tank. Measure the sending unit from the mounting flange to the float in the full drop position, then measure the depth of the tank from mounting flange to the bottom. The measurements should be close to each other. If they aren't then the travel is not right for the tank. I would think the vendor or someone out there could give you the measurements and angles of a known good stock unit. Unfortunately both my cars have fuel cells and I don't have any idea what the measurements might be.