If the tach is really out that much, it should be replaced. In Canada, a 5% error is the most which will be tolerated to be placarded. Such errors can allow engine or propellr speed limits to be exceeded in some cases. Not good. They can also really make some performance charts inaccurate.
If in doubt, have the tach check optically - it's easy. Generall older tachs cannot practically be adjusted, as their cases are crimped shut. Buying a new tach is generally the only soultion.
Pilot DAR