Crap. You're right - that's what happens when you go from memory. Thanks for the correction. Apparently he's been flying at 83% of whatever the RPM is set to for Tach. Sigh - don't get old.
Not necessarily. He could be cruising at exactly the screens cruise rpm setting, and the tach will still be less than the Hobbs.
The Hobbs counts running time at 100% of actual time passage - one clock hour is one Hobbs hour. The Tach time is a function of rpm and time - so idling at 800 rpm, with a normal cruise rpm setting in the Skyview of 2400 rpm, the tach will only advance 20 minutes for every hour of actual idle time. Since we all idle during startup and shutdown, and don't taxi at full rpm, we all will have a lower tach than hobbs time.
The tachometer reading is historically used for maintenance purposes - based on number of engine revolutions. Turning the engine slower than the "rated rpm" will turn the tachometer slower. Otherwise we would all just use Hobbs time and there would be no need for tach time.
From the Skyview Users Guide, section 6, revision K
Tach Time – labeled TACH when displayed on an Engine Page, is a measure of engine time
normalized to a cruise RPM. The cruise RPM parameter must be set properly in the system
setup to generate a correct tach time.
Hobbs Time – labeled HOBBS when displayed on an Engine Page, is a simple timer that runs
whenever the oil pressure is above 15 PSI or the engine is above 200 RPM. It essentially runs
when the engine does.
The tach time is at 100% of the time in cruise based on nominated ( in Setup menu) set cruise RPM. If you set it as say 2600 rpm but decide to cruise at a lower rpm then the actual TACH hrs will be an artifact of that, in other words lower. I’m not sure what the case would be if you fly at a higher rpm. The Redline rpm doesn’t have any impact on that 
.
