engine tach

Is the engine tach somehow tied into the main wiring harness for the other gauges? I know my volts, temp, etc are a bit off either high or low and am wanting to run fresh ground wires to all the gauges. But here is my question, my tach reads low I think. When I am in gear it reads about 500 or so. But if I adjust it to 750RPM or whatever for forward idle it goes into gear kinda hard and my speed is kinda quick when in idle zones. So i am wondering if my ground problem is reflected in the tach too? I have the ProTec system.

I have tried to adjust the idle speed as low as possible because we have some idle zones and I still manage to kick a wake and the stupid fisherman yell at me.

So, is tach tied into the rest of the dash grounds?

Thanks for all your help!