NOS tach problems

Ah Yes! Just re-read your post. Dixco made a lot of the hood mounted Tach's back in the day. I remember having to play with a handful of resistors once on an old Sunn in a Mustang. I believe I had to raise the resistor values until I got it to match my Shop Tach. Can you get your hands on some resistors with different values? Possibly radio Shack? Can you post some pictures of the resistors you have and the Tach itself? I think Just maybe if you put another resistor right in-line with the wire to the coil you could fix it. I'm Guessing there's 3 wires, 1 for light, 1 to coil negative & 1 ground. I'm not an expert just thinking out loud. I wonder if the Tach originally came with some extra/spare resistors.