This is the second Pontiac rally gauge tach I have come across that has read on the high side. I believe the problem is the shorting bar that is pressed into whichever hole you require for your application, be it 4,6,or 8 cylinders. It is a plastic pin that holds down a small piece of spring steel that must get weak over time and not make good contact, causing the tach to read too high. My fix is to remove the bar, and solder a 1/2" bit of wire right across the gap making a solid connection. This cured both tachs, so I'm guessing it's the way to go.
