when i had my car running while the dash wiring harness was unplugged, i came to the question: how do i know if my tach works properly and is reading the right RPMS (that is, when it is plugged in).. its a fairly old car i am not sure if i maybe need to look into that or not..

I have a little handheld meter that measures the RPMs, but then you have to assume the meter is accurate. It hooks to the coil. I've seen meters for sale that you just point at the rotating object and you get an RPM reading. Don't know how accurate those are. You could also count how many times per minute the timing mark passes the pointer, but that method really tires out my eyeballs and I can only get an accuracy of + or - 50 RPMs. :laugh:

I have an old dwell/tach meter. I'm assuming that it is fairly accurate. It reads very close to what the tach reads in all three of my Datsuns. The truck's tach is not factory and can be calibrated but I've never heard of a procedure to calibrate the factory tachs.