[I did do a search before asking this...]
OK - I just discovered what the difference between a tacho and a Hobbs meter is
Now I have two questions:
Is there a standard setting for the revs at which one tacho hour equals one clock hour? (C172N)
If the tacho is set to 100% at say 2400 revs, is it fair to assume that it ticks at 20 seconds per clock-minute at 800 revs?