my original gauge shoots past full and does not move at all.. i am trying to find out which resistor to use to bring that needle down anybody have any ideas??????????

 

I don't know that you can use the original gauge with a Mustang sender unit even with a resistor. It's not a voltage issue since both were 12V. The gauge and sender need to match. If you are intent on using your original gas gauge then you might try to figure out a way to use the original sender in the Mustang tank. Otherwise, you need a fuel gauge that is compatible with the sender you have in the tank now...