Bared wire, perhaps?
I may be wrong, but the way that I understand gas guages is that the sending unit provides the ground, and that a rheostat is used to provide variable resistance.
When the tank is full, the rheostat is in the "fully grounded" position offering zero resistance, and the guage reads "full"
At 1/2 tank the rheostat is only supplying 50% of the resistance value... etc
So my theory is that if the wire leading to the sender has been chafed and grounded before reaching the sender, it will always display a "full" tank.