if it's a 6v hour meter, and you run it on 12, if it's mechanical.. it will count too fast.
running it thru a resistor is dubious as resistors drop voltage based on load. IE.. there is no '6v dropping resistor. ).
you COULD build a vreg out of a couple radio shack parts and feed it and be fairly accurate.. or get a 12v meter from napa like I did for my 951 :)