I'll give ya'll a little secret I figured out years ago. My first vehicle was a 53 Chev 3100 in 1990.
Here's how you make the gas gauge 100% accurate on 12 volt. Converting to 12V from 6V doesn't require a resistor. IF you don't believe me, remove the fuel sender after you install a resistor in the sender to gauge line. Move the tank sender up and down and it'll be innacurate.
SO, do this. Cut the sender line under the dash. Temporarily install a very small light bulb (like another instrument panel bulb). Operate the sender up and down. You may have to try a few different bulbs until you find one that is 100% accurate. This will work. You will actually notice the bulb will glow a little. It won't fully light up. It's the only thing I found to actually work. Solder the wires to the lightbulb in series with the sender wire when you find one that makes the gauge read E when down and F when up. Reinstall the sender in the tank.
One word of CAUTION. When the gauge reads E, you are not low on gas. YOU ARE OUT!! NO JOKE. Ask my sister. She had to help me push the truck into a gas station.
