HELP!!!! My tach is reading too high

I have a dixco illuminator tach. But I tried the sunpro tach that I was going to use. My coil is really close to the wires and the wires are about a year old. I haven't been able to check it out today cause I feel a bit under the weather but I will try wiring it to the grey wire first. That might help cause it won't be close at any of the spark plug wires. Thanks for all this info. I'd be lost without it.