Sweep needle tachometers are notoriously inaccurate on Rotax engines. Both the two strokes and the 912 series.

I had a VDO and a Rotax tach in my old plane that were both terrible. I replaced the Rotax with a digital tach from ACS that was great. Spot on compared to an optical tach.

I have a Dynon panel in the Mangy. That checked perfect to the optical, too.