GM-GMC: gas gage not working ., chev truck, variable resistor

Question
1975 chev truck,when I fill it up the gage go's way past full,when it shows I have 1/4 tank it is empty.Thank you for any help you can pass my way.

Answer
Well, it is a pretty simple system, however diagnosing it can be a little tricky.  Variable resistor in the tank, some wiring and a gauge on the dash.  Typically the resistor is part of the sending unit and overtime they have residue build up on them or they wear due to the constant up and down of the fuel in the tank, this causes them to change ever so slightly or in your case a whole bunch.  The gauge will simply move the needle depending on the voltage that it receives from the sending unit.  If it is receiving a higher voltage it will display a higher reading.  That being said, generally gauges have small stops on them which limit their travel, I cannot recall ever having a gauge out of a 75 chev, so whether they have them I can't say.  One option is to unplug the fuel tank and sending unit, turn the key on and see what the gauge displays.  It should be reading empty (we unplugged it so no voltage should be going to it)  If it reads 1/4 tank, I would be inclined to think the gauge has jumped or shifted or malfunctioned in some way causing all the readings to be off.
Sorry to be long winded, there is a device at the dealership called a signal generator.  What it does is it reproduces the voltage that the sending unit does from inside the tank.  It is plugged into the harness instead of the fuel pump and sending unit and there are a series of dials etc that you can slowly adjust the voltage and the gauge should move accordingly.  Most dealers don't like to lend out tools but if you ask nicely you never know.
Hope this helps