As Phil indicates, the sender/flange is SUPPOSED TO BE GROUNDED. The flange screws are SUPPOSED TO GROUND THE FLANGE of the sender to the tank.
The center tap is isolated and is NOT SUPPOSED TO BE GROUNDED. It's wire goes to the gauge. Many times the flange does not get good ground due to the gasket on the flange and rubber pads on the tank, rubber body mounts, rubber motor mounts......all trying to circumvent the ground path and have to be "jumped" with braided straps (which must be viable). There are many connections along the way to get corroded or broken. Many times you need to add a wire under one of the flange screws to the frame or body or battery.
If the center tap, or it's wire, touch ground, the gauge will not read properly.
I assume the gauge works with under bed tank/sender. I assume that both tank senders and gauge are 0-30 Ohms.
Gas gauge/sending is just about the number 1 electrical issue on Stovebolt. Like most electrical issues, it's usually the ground path. It's a long trouble invested path.
Yes, I believe the tank was under bed in 1948.
