In my 48 Chevy pickup, I used all the guages but one on the 12 volts without a problem. The guages are in a box where they have been for ten years so I can't remember exactly what was what. But the fuel guage for sure worked on 12v the Oil was mechanical, the amp guage worked, (just didn't go as high as with the 6V) and the water temp was replaced with a S-W INSIDE the original cluster. It did go backwards because the thing was up side down in the cluster. But I just remembered that and it never bothered me.

Brian