[19Fordy]

Gezer, I just spent the last 2 weeks trying to get my 1940 OEM gas gauge to read accurately. Finally got it to read accurately. Here's what I did. Disconnect + battery terminal and switch OFF ignition. Disconnect + wire and ground wire from tank sender. I have a plastic tank so I am running a separate ground wire from sender to frame.

Lay a piece of cardboard on trunk floor and place sending unit on that cardboard so it touches no metal. Stuff rag in open hole in gas tank. Take one wire with alligator hooks and attach one end to power wire that feeds the sender and clip the other end of same wire to input screw on top side of sender. Take another wire with alligator clips and attach one clip to bare metal of floor trunk latch hook and clip the other end through one of the screw holes in sending unit. Make sure all these wires are making good contact and that the sending unit is insulated from the car with the cardboard. Now reattach your battery cable and switch ON your ignition.

Move your float all the way up to the FULL position and check the needle on your gauge. Give needle a chance to heat up and register as the King Sealy units don't operate on Ohms law. It may take a minute, etc.

Then move float its lowest position and check gauge reading. Keep in mind that when your gauge needle is on the E mark you want to have enough gas left in your tank to get you to the filling station. If your fuel gauge doesn't read correctly you can make adjustments by bending your float arm and also by adjusting the star wheel inside the sending unit. I did both since my plastic tank holds 16 gal. not 14 like the stock tank.
Be sure to flip your ignition switch to OFF when you connect and disconnect you alligator clips when making adjustments.

To adjust your tank sending unit, you turn the starwheel inside the unit clockwise to make the needle read closer to the Full mark and CCW to make it read more towards Empty. You adjust the starwheel with a small flathead screwdriver moving it only about 1/32 in. at a time with the tank sending unit clamped in your vise. A brake adjusting tool is used to pry off the top of the sending unit and you can use channel locks to carefully reinstall it.

This entire process is very tedious and you must be patient as you will make incremental small progress each time you adjust starwheel. When you get tired, STOP and take a break so you don't do something stupid out of frustration. I disassembled the fuel gauge but did not adjust it as it is extremely delicate.

Gezer: The first and easiest thing I would do in your case is remove the sending and rotate the startwheel CCW just a teeny, weeny bit and reinstall it to see any differences in needle reading. Also, make sure your float is not cracked or filling up with gasolene. That will effect gauge readings.

Once you get the gauge to read correctly reinstall the tank unit using your OEM wires. It should read the same. If it doesn't your OEM wiring may be faulty.

Even with the original steel tank, I think it's a good idea to run a separate ground wire from your sending unit to your car frame just to make sure you have a really good ground.

Hope this helps. More experienced electrical folks may have a better approach.