if you don't have an extra tachometer for calibration, set the VDO tachometer to pulse mode ,then set p=97. That would be OK. The flywheel of Yanmar 3GM given 97 pluse per rev.to the sensor then to the tachometer as reference. or you can go to the adj mode to cal by an individual tachometer . Yanmar has a technical bulletin no. MSN 03 -015 telling the procedure for PULSE mode setting