How to adjust the tachometer?

[JohnMc]

I have not removed it yet but wondered if there was a way to adjust the tachometer without destroying it?
Mine is about 400 rpm off and would like it a little more accurate if possible.
Thanks
John Mc//

[kmead]

There is a potentiometer inside the housing, some have drilled a hole in the housing to be able to access it while the unit is running.

[Jon124]

Try cleaning the ground connections. Mine used to slowly increase rpm at constant speed as the gauge heated up. After cleaning the connections the tach always reads the same rpm at a constant speed.