Engine Hours Vs. Miles: Which Matters More For A Used Diesel Truck, According To Drivers?

The mileage of a vehicle is the standard way to judge how much it's been used, and it's one of the few main metrics most people pay attention to. For commercial vehicles and diesel trucks, engine hours are also very important since these vehicles don't wear only while driving. Because of this, many diesel truck owners estimate idle hours in terms of miles. A few owners on forums noted that each idle hour is equivalent to 30 to 35 miles.

For commercial trucks that spend most of their time idling, engine hours are arguably more important. This is because excessive idling exerts a lot of wear and tear on the engine and is one of few common mistakes that can ruin an engine. On the other hand, for trucks that spend most of their time driving, it's the other way around. One is not necessarily more important than the other, it all depends on how the truck is used.

Mara Godding, a technical information manager at Alliant Power, told EngineLabs: "If you're buying a truck, you need to look at engine hours..." She went on to say "That will tell you if there's lots of idle time or lower average speed, like on a plow truck. Wear and tear on an engine is based on hours run, not just miles traveled."

Some trucks are used as delivery vehicles. Others are used as plows trucks, tow trucks, and service vehicles. It's also standard practice to use a diesel truck to power refrigeration units, lift gates, and hydraulic tools. In all these environments, the engine is running, but the truck isn't accumulating any miles. If scenarios like these happen frequently, the truck may show a relatively low odometer reading even though the engine has experienced much more wear.

This is why some commercial and heavy-duty trucks are legally required to display engine hours to comply with regulations. The importance of displaying engine hours is one of the things you should know before buying a diesel truck. For example, one owner on Reddit reported having a 2013 GMC Sierra with just 78,000 miles on the clock. This does not sound all that high mileage for a diesel truck, but he also mentioned that the truck had 2,858 engine hours. 

If a decent portion of those hours were spent just idling, using the previous estimate that one hour of idling can equal around 30 miles of wear, the engine could have experienced the equivalent of tens of thousands of additional miles, so to say. Moreover, if the truck has an unusually high number of engine hours but a low indicated mileage, it may also indicate possible odometer tampering. 

Since most people view maintenance and service intervals either by mileage or time passed, it's also worth looking into engine hours, especially if your truck idles a lot. Some manufacturers list these metrics in their maintenance guides. For instance, the Cummins ISX15 maintenance guide schedules maintenance tasks both in regards to time passed, the mileage, but also engine hours. 

According to this guide, one engine hour equals about 30 miles in severe-duty use, 50 miles in normal-duty use, and around 70 miles in light-duty, usual highway use. According to Ford's Power Stroke maintenance schedule, one engine hour roughly translates into 25 miles of driving. By relying on these metrics, you can schedule maintenance tasks more effectively.

Diesel engines have their own specifics and essential maintenance tips you should know, and changing the oil is typically done every 5,000 to 7,000 miles. However, if you've driven your Ford 3,000 miles while also accumulating 80 or 160 hours of idling, you have effectively reached your oil-change interval although there is 3,000 more miles on the odometer.