"Robert M. Gary" wrote in message m...


My aircraft (Mooney) came from the factory with one. Some argue that
its actually the legal way to count 100 hour ADs (since the tach
doesn't give 1:1).

It is a legal way. The FAA says "hours" and absent any specific instruction
from the manufacturer, you can use the recording tach, a elapsed time meter,
or some other reliable record. The FAA just asks that you be consistant.
However, I don't think they'll buy the TACH multiplied by fudge argument.