Just as a data point, BrianK replaced the caps in my cluster (1992, 137k miles) and:
Tach is accurate to 5,000 rpm
Tach shows 7,500 at 8,000 rpm actual
Speedo is accurate to 40 mph
Speedo shows 75 mph at 65mph actual
It's going to need some adjustment on the pots, which were untouched prior to...