If you have ever operated or been around heavy equipment, you may have noticed there is no speedometer. There is, however, an hour meter. This meter is what is used to determine service intervals for the machine. Most trucks are equipped with an hour meter. Why? Engine manufacturers have determined, through much testing, how often the engine needs to be serviced. This interval can be measured in hours or miles, depending on the duty of the equipment.
Let's say you have a concrete mixer operation. We know the engine is going to run for hours a day, mixing, driving, and unloading. Lots of idle time, yet not many miles. If the truck engine started at 07:00 every morning and ran till 18:00 every evening, that would be 11 hours each day. During that 11 hours, you drove 100 miles from the batch plant to the construction project, say five days a week. It would take a very long time to reach a 35,000-mile oil change interval.
This is where the hour meter steps in. It will record all hours from start to shut down. Until electronic engine controls came along, engine revolutions were monitored by a mechanical tachometer ( tach ), cable-driven off of the engine directly. In most cases, there was an hour meter built into the tach. This is where an hour wasn't an hour. An engine hour isn't a measure of time; it is a count of engine revolutions. Depending on the operation R.P.M.'s ( revolutions per minute ), one engine hour could take much more or much less than a clock hour.
The tachs were marked for each manufacturer, but in most cases, one engine hour was 100,000 revolutions of the engine crankshaft. Ten hours at 1,000 RPM equaled one engine hour on a mechanical tach. I will say, this was on engines that were designed to run at up to 2100 rpm, much different than today's engines. Also, service intervals were much shorter, 10,000 to 15,000 miles, depending on the owner of the truck.
Today's hour meters are electronic, still measuring engine revolutions but in a different way. For our cement mixer, it would be considered severe duty, service intervals are 35,000 miles or 750 hours whichever comes first. That would be as if you averaged about 45 miles per hour, hard to do running 11 hours/100 miles a day. If one were to change on miles, 500 miles per week would be 70 weeks to get to 35,000 miles. That's over a year. Servicing on hours would be 13 - 14 weeks, assuming 11 hours per day, 55 hours per week to get to 750 hours, that sounds better to me.
Short-haul trucks, the local delivery operation, 45,000 miles or 1,000 hours, still an average of about 45 miles per hour. Long haul and efficient long haul are 60,000 and 75,000 miles, respectively. No hours are given, but if you idle for temperature control ( A/C - heat ), I would recommend watching your hour meter and service sooner.
Be safe out there.