My TC33DA has an hour meter that goes by clock hours. Regular time. If you idle for 100 hours it will show 100 even though hardly any wear and tear. The TC55D has an hour meter that shows an hour at 1830 RPM's which is a good average rpm and would more accurately show when oil change is due. My old International 2544's meter shows an hour at 540 pto RPM so 100 hours on that is much more than anything else. I wish there was a standard. i stated this before years ago. Should be a standard way to show an hour. I really think the TC55D has it right with a good average rpm. I dont like clock time unless im charging someone by the hour meter
Mike
Mike