So if I understand correctly, you were running the 6v gauge on 12v?
Yeppers. Original sending unit. It's been a while but I believe the reading was a tiny bit different, the width of the needle, but I don't remember whether it read a tad higher or lower than before. Either way it wasn't an issue. When it read in the neighborhood of 1/8 tank it was time to visit the gas station.
