My favorite all time best way is to put a 6.8 volt Zener Diode (stripe facing away from the gauge) right at the terminal and be done with it. DigiKey part number CZ5342B CT-ND for $ 0.89 each.
This way there are no electronics to burn out (other than the passive) and they are small enough to fit right behind the gauge. A barrel splice on one end and a terminal end and then heat shrink over the whole thing so as not to cause a short.
