That thread is about the older tachs. The newer 90-92 instrument cluster has different gauges and uses a bit different technology.
The tach is calibrated by precision cut resistor module that's soldered onto the driver module board in the instrument cluster box right below the tach.
It is not easy to adjust but if one has access to the right equipment (Fgen, meters, scope, parts), it's not impossible. I myself am thinking about adjusting mine (got sold a dud that shows way off) but can't find the time. If I succeed, I might offer this service to fellow 3rdgenners.
Hope this helps.