I have the same problem with the fuel gauge on my 52 GMC. It reads 1/3 full with an empty tank and 2/3s full when the tank is full. I have disconnected with wire from the gauge to the sender and the gauge reads full and with it grounded directly to the battery it reads 1/3 full. I put a temporary jumper from the cluster case to ground and it still reads 1/3 full. The sender checks good on the ohms resistance through its travel. The electrical system has been converted to 12 volts and I am not using a voltage drop resister to the gauge. I tried a "Runtz" type resister and the gauge would only go to the 1/2 full position with a full tank and still not go to empty.

I really would like to get the gauge to read empty when its empty. Does anyone have any other thoughts other than I just have a defective gauge?