[dsraven]

A fuel gauge is basically a needle held between electromagnets. One magnet is a constant and the other is variable depending on its ground provided by the sending unit. If you remove the wire from the sending unit the manetism from the variable side will be gone so the needle will be drawn to the manet that is constant. That means the gauge will read all the way over to empty or full, depending on which side the constant magnet is placed when thegauge was built. If you ground the sending unit wire the gauge should pin thd opposite way. Doing this would test the gauge opetation. Now, with that said, you can see the importance oc having a dedicated ground to the cab and also from the tank sending unit to the cab. Otherwise the ground from the sending unit to the cab relys on the tank touching the cab somewhere. It is supposed to be sitting on some rubber or othet kind of mount so getting a good ground to the sending unit could be a challenge, relying mostly on the tank straps. Possibly a dedicated ground strap from the sender to the cab sheet metal and also from the cab sheet metal to the truck frame, engine block or battery would help you get the gauge to read accurately. Of course there are other reasons why the gauge could read wrong, like a poor float or a bad sending unit, but dont forget the othet part of the circuit. The circuit is only as good as its weakest link.