Can you think of a legitimate reason to modify a tachometer in this way, as falsifying one's hours (e.g when working to a commercial licence) is the only reason that currently springs to mind?
Nope. Who uses the inaccurate hours count from a tacho to log their hours no-one I should think. Six hours difference!!!, so logging six hours against a one hour flight I don't think so. Bad enough using a Hobbs for logging flight time let alone a tacho.
So long as the revs are accurate, forget the hours on it, there irrelevant..