Have you tried reaching out to VDO or whatever brand your tach is? Sounds like it might be more of a factory tech service situation than a DIY. That said, did your tach ever work correctly? If it’s always been wonky, there may be calibration switches on the back, but you probably still need mfg’s input to set correctly. Rambling further, if there is a switch for no. of cylinders, be sure it’s set right.