I wired up a tach in my car a long time ago and never used it because the tach I was using was actually broken...well, I got a new one the other day and hooked it up to the previous wiring and it seems to work, except for a problem. When holding at a steady RPM, the gauge will read ok, but when changing more than a couple hundred RPM's, the needle goes up and down spastically, I bet if I took the average value of where it was moving around, that would be my actual RPM...anyway, when I'm at a steady half throttle or whatever, I suspect it could be reading lower than true, but I"m not sure. What do you think the problem is?