A bud stated that the hour-meter on his 2005 Volvo BL60 TBL recorded hours in straight time regardless of RPM, even at idle. The hour-meter on my Case 580M does not. It records hours at a higher rate when excavating than when loading, I assume because the engine is operating at a higher average speed. I know mechanical hour-meters on some old tractors operated off the tach, and the hours were RPM dependent. The hour-meter on my ZTR mower only operates when the mower deck is engaged. How are hours measured on current construction equipment? That would make a difference when evaluating used equipment.
An hour meter is the last thing to rely on when looking at used equipment. Some of the newer machines are probably better because the hour meter is part of a larger display and can't be changed or tampered with as easily. I think a lot of machines with a separate hour meter count hours as soon as you turn the key on. Cat and probably others also had the hour meter connected to an oil pressure switch. They would only count when there was oil pressure.
I've bought almost all of my high-dollar (for me) equipment used over the years with mostly good results. Every used machine tells a story. I always like to see if the wear matches the hour-meter or odometer. I'd also like to confirm that my hour-meter is working correctly for equipment maintenance. Currently, it averages about .6 hours on the meter for every clock hour the engine is running.
Personally, I wouldn't rely on it. How it run's, how it shift's, and how it speak's to you is what I would rely on. Just my opinion.
The hour meters that are electrically powered clocks, record time based on when power to the meter is turned on. They are not based on engine speed or anything else. You may find machines with hour meter readings on a monitor screen and you will need to check the operation and maintenance manual to see if operating time is counted differently. You may find machines with mechanical hour meters run from a gear train with a speedometer cable or something that will count operation that is variable with engine RPM. These types of counters would only be found on something very old. Old farm tractors usually have these types of counters.
Mechanical tachs and hour meters will usually say "hours at 1,300 RPM" or whatever speed right in the center of the tachometer. That's actually counting every single rev the engine makes.