why does my tach read 2x engine speed?

On both my 91 OCCs the tach is wrong. In my driver best I can guess its double what I'm turning.
5200RPM indicated at 73mph, etc.

On the other OCC It idles around 5K on the tach and buries over 7K when you give it some gas.
I don't think either of them were doing this when I got them......

The circuit board on the back of the cluster is toast. I had a tach constantly go the max, and it was the circuit board. The board from the 91-93 RMW and I believe the 91-93 RMS will work. The only difference is that you would have to add the gate ajar bulb to the RMS board. I might be able to get an RMS board next week. I have a couple of RMW clusters, but do not want to tear them apart.

good thing I have an extra OCC cluster then got a pair of RMW clusters too (I'm a hoarder, lol)

Ha! I have an extra OCC cluster, two RMW clusters, a 94 RMS cluster, and a 96 Caprice Sedan cluster.

I would replace the board with the RMW board. The OCC clusters are harder to find.

The board is easy to replace... remove the screws and gently pull it off of the pins. Reassembly...push over the pins and replace the screws...if I remember correctly, the tiny screws have dual threads. One thread is more prominent than the other. If you put the screw in with the more prominent thread in the path cut by the more shallow one, it will strip out. the way you can tell if you have it in the correct thread is that the screw will thread in easily. If you put it in the wrong way, it will have resistance to screwng it in.