Originally Posted by 52Carl
Does the gauge peg with the ign switch on?
[1] That usually is a symptom of no ground from the top of the sending unit.
[2] 12 volts to a 6 volt gauge should not damage the gauge or sending unit, it just won't read correctly.
Carl
[1] Yes
[2] No damage to the sending unit (simply described as a variable resistor); but, the 6v gauge might be wounded/fried.

A 12v to 6v runtz-style reducer on the input side of a 6v fuel gauge is recommended by most people.