John,
If you are interested, and for the sake of accuracy, the enclosed link provides the blow by blow operation of the fuel gauge.
It doesn't appear to function at all like a ammeter, and does require a case ground to function. The sending unit path to ground is one of two. The difference between two ground paths is what causes the needle to move in either direction. This is how I think it works.
http://chevy.tocmp.com/1955truck2shop/55ctsm1208.htm Stuart
