So, played around with this a bit. Decided to test all 4 tachs I have and see how they differ.
The 4 clusters I have are
Original to my car white face 140MPH
A good spare 140MPH original cluster
My 160MPH cluster
A junk parts cluster. This one I played around with the pot a bit to adjust
Here's how it plots out. The RED line is the standard set value for the RPM. Closer to the red line, the better.
As you can see, the junk tach that I played with last night is the closest after tweaking the pot a little. The 160MPH and 140MPH cluster were pretty even. The surprise was the white face cluster. This is the cluster that's been in my car since 1988 and the one i've been staring at for 20+ years. It's off. WAAAAY off. It's one of the reasons I installed the Autometer shift light you might have seen in earlier posts. The shift light would go off at 5400RPM and my tach would be showing almost 7K.
I've also discovered the tach in my 160MPH cluster is a late 1989 tach with 2pots for adjustment. The earlier 87-early 89 tachs only have a single pot. My next step is to get that tach (blue line) as close to the red line as i can.
Anyway, too much geek stuff i guess but i ripped the shift light out, so i want to get the stock tach as accurate as i can in the 5000-6000 range.