[mattswb4x4]

i installed a 4o gal tank in the rear of my truck today and when i hook up the sending unit wire it reads half full , but it was bone dry so i put 5 gal in it and it reads 1/4 past full. is it a bad sending unit or are thay not compatiable with are trucks guages? my truck is a 71 and the tank is out of a 84 blazer. any help please!