fuel tank sending unit

[rudders_midship]

Last year I had fuel tank built by SP tanks the tank came with a sending unit installed it has a shaft and float assembly (vertical) the problem is the gauge will read full when full, but about 4 or 5 gallons aprox into a trip it goes to about half and when reading empty it still has about 21 gallons left in tank (50 gal tank). Should I go to the more conventional sending unit (rod with cork float) or is my gauge not compatable with SP type sending unit. Any help would be helpful since boat is in my shop ready to do minor fixes and improvements.