I seem to recall posting on this subject before.
There can be quite a bit of variation on the hour gearing of some tachometers.
For instance, the standard fitting on the PA28 is 2300RPM, but for Cessna 152/172 its 2566RPM - about 10% different.
You can find a listing on
http://www.aircraftspruce.com/catalo...mech_tachs.php
But whichever you use the Hobbs time will usually be the biggest number. Thats why hire aircraft use them, the operators would have to quote a higher hourly rate otherwise. I personally advocate tacho hours for group owned aircraft, much fairer overall.