CRAZYBROADSWORD,
I don't believe that the tachs run off the mags. The governor makes adjustments in RRPM using shaft rpm (completely separate system from the tachs). I could be wrong, but there are small magnets built in that allow the governor to calculate RRPM on it's own (simple - but effective). IntheTin is correct in the fact that the tachs are on a different circuit for safety. For example, if you have an ALT failure + GOV failure... you are still good to go (but tachs won't last long on battery alone). What will happen if you lose your tachs and nothing else in a normal situation?... absolutely nothing. The governor will compensate for you and you can land safely without low RRPM or overspeeds. The need to expedite a landing is predicated on need power for other items (lights, radio, transponder, power for an emergency in-flight restart, etc).