Tachometer not running full range

[eternalphoenix64]

Having an apparent issue with my tach. Presumably the issue has been present since I bought it less than a year ago (not my daily driver and had a bit of an electrical gremlin in fuel pump circuit project for the first 6 months of ownership).
2.9L, 4x4, 5spd

At low rpm, the tach SEEMS to be accurate (I haven't done any testing yet). Once I get above ~2000 RPM the needle seems to stall. By "stall" I mean that I am still accelerating, and the needle still moves, but it does not seem to move at the same ratio as what I'm hearing, especially compared to lower RPM.

I have tried to take the truck to redline based on hearing (around 22-25 mph in first gear) and the tach only read around 3000 rpm.

So the questions I have
1) good test method to determine the root cause
2) how to fix it depending on what I find

Any help is appreciated.

 

[steveoelo]

Does it seem to get "stuck" at certain rpms? Mine did that so first I checked all the wires on the distributor then pulled the cluster out, took the tach out a re soldered the solder joints on the back. Been good ever since