Tach suddenly reads too high?

[SailingHarmonie]

Here is one that has me scratching my head.

My Volvo TMD22 with an alternator driven tachometer. This is the OEM internally regulated, isolated ground, 12V alternator only used to charge the start battery.

In the past couple weeks the tachometer has started reading high by 25 to 50% The error is not consistent. No work has been done on the 12V system, the alternator, belts, or engine instrumentation. The alternator seems to be working fine, although I haven't done a deep dive into it other than watching the voltage when the engine is running.

I can come up with lots of reasons a tachometer of this kind might read LOW. But what would make it read high?

[GordMay]

Analog tach’s often have a rotary calibration switch and potentiometer, accessible on the back.
Over time, the contacts, in the switch, get dirty or corroded, causing the meter to read high.
To fix this, use a small screwdriver to move the switch, back and forth [between settings], a few times, to scrape off the corrosion. The switch may be hidden, behind a rubber plug.