Cliff,
I bought my '49 pickup when I was an undergraduate electrical engineering student umpteen years ago. As a project for a measurements class, I setup a '49 fuel gauge and sender and then measured the current into it as a function of the voltage applied.
The system (gauge + sender) is designed to read correctly independent of the voltage applied to it. If it wasn't, then the gauge would be moving everytime you revved the engine and the generator voltage went up.
The only question then is how much voltage can be applied before it does burn out the coils. I can't remember the exact numbers from my test but the conclusion was that the system didn't draw enough power at 16 or 18 volts to even worry about.
The summer after that class, I rewired my '49 and converted to 12 volts. I ran 12 volts through the '49 gauge without problems for over 20 years. I did have one fuel gauge problem. The sender in the tank finally broke (mechanically) after 45 plus years and I had to replace it.
I've posted this here before and lots of folks have horror stories about how their gauge fried on 12v etc.. If you hook it up wrong, even at 6 volts, it'll cook in a heartbeat. That's why the divider between the terminals on the back of the gauge.
I'll have another look for the paper I wrote for the class, it has all the readings, graphs and conclusions on it.
