Is there any way to tell the difference between a 4 cylinder Tachometer and a 6 cylinder Tachometer, that are removed from the original vehicles? Thanks for the help on this.

yes.

I was running a V6 tachometer in my 88 Coupe 4 cylinder for a while and then swapped the circuit panel on the back to make it read properly.

if you install a V6 tach in a 4 cylinder cars (fieros) it will read about half the rpms (I.E. redline is 3000-3500 or so)

a v6 tachometer has a small diode that makes it read differently than the 4 cylinder tachometer.

I have to fix my tach again today along with some electrical bugs so I'll take pictures of the difference between the 2 types if it will help you out.

Thanks! Yes, photos would be great.
I have two different Tachs, out of cars, that i would like to try and determine which ones they are.
Thanks again for the help.