Most of us are accustomed to looking at the mileage on an odometer when we evaluate the lifespan of a vehicle — especially its engine. Anytime we purchase a used truck, one of the first things we look at is mileage. Mileage is a generally accepted form of understanding how much wear and tear a vehicle (and its engine) has endured.

The problem is, in some cases, the mileage may not be a very good indicator of how much use an engine has actually seen. Especially in the case of diesel trucks, trucks that spend a lot of time idling, or delivery trucks that spend an excessive amount of time in the city realizing stop-and-go operations. With these trucks, engine run time is a better gauge of how much wear an engine has been exposed to. And for those types of vehicles, engine run time is a better gauge for scheduled maintenance intervals.

Vehicles like this crane truck often operate their diesel engines for extended periods while the drivetrain is static, and the odometer is not clicking off miles. That means maintenance intervals should be tracked by engine run time versus accumulated mileage.

To understand the difference between engine run time versus mileage, we spoke to Mara Godding, technical information manager at Alliant Power. Alliant Power distributes aftermarket, new, and remanufactured diesel fuel injection and engine components for medium and heavy-duty engines.

The folks at Alliant Power know what parts of diesel engines need replacing, and when they need replacing because they are the people who distribute these parts. “If you’re buying a truck, you need to look at engine hours,” Godding says. “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.”

Oil Changes Ain’t What They Used To Be

According to Godding, if you have a pickup truck with a diesel engine, you need to change the oil every 7,500 miles, which would equal 250 engine run time hours. This information is actually in the vehicle’s owner’s manual. Yes, the owner’s manual, something very few people actually ever read.

If someone did actually read the owner’s manual, it would tell them to complete maintenance on diesel engines, both at 7,500 miles and/or 250 engine hours. This calculates to a ratio of 30:1 (7,500 miles divided by 250 engine hours). “One hour of engine idle time is equal to approximately 30 miles driven,” explains Godding.

This Chevrolet pickup truck has an hour-meter, which can be brought up on the dash display. Based on the calculations provided by Mara Godding, one hour of idling is equal to 30 miles of driving. This truck with 1,526.0 engine hours and 76,195 miles hasn’t spent much time idling at all. In fact, it appears it has spent most of its time on the freeway, making this a good used truck to purchase. Everybody knows “freeway miles” are the good ones.

The data for engine run time on modern vehicles is saved in the vehicle’s ECU and is easily accessible to view through the dashboard display. Simple navigation through the different dash menus will bring up the engine hours screen. If you can’t find it, look in the owner’s manual for instructions (I know, I know, nobody reads the owner’s manual).

Just like vehicle mileage, this data point cannot be reset to zero by the user. Engine run time hours are a crucial measuring stick to understand how much an engine has endured, especially for diesel engines. According to Godding, the types of vehicles where this would really make a difference are, “Tow trucks, bucket trucks, anything with Power Take-Off (PTO) use, school buses, any vehicle that has a lot of high idle-time, or just runs at lower speeds, like an in-town delivery truck.”

A diesel truck that spends its day doing stop-and-go deliveries around town and never gets above 25 to 30 miles per hour runs into a situation where exhaust temperatures don’t get high enough, and you can end up with carbon build-up. This is not good for the engine and requires a more frequent service schedule.

PTOs require constant charging of the battery to operate and thus accrue more time idling, which will not show up as mileage. For those trucks that spend their days idling or staying below 30 miles per hour, the lack of exhaust temperature can cause havoc. “On a Cummins ISB engine, it won’t run a DPF regen if not over 30 miles per hour,” according to Godding. A regen is needed when your DPF (Diesel Particulate Filter) begins to fill with soot.

Sensors in the filter will tell the engine computer “time to do a regen,” and the fuel injection will put fuel into the exhaust. In turn, during the exhaust stroke, the fuel will turn soot into ash. But, if the vehicle doesn’t get over 30 miles per hour, the Cummins engine management won’t run the regen. “It is important to put about 45 miles of freeway time on these once a week to complete the regen,” says Godding. Long story short, idle time can be bad for a diesel engine.

A truck with external equipment that requires the diesel engine to run in order for the equipment to operate will generally have engine hours much higher than the vehicle mileage may indicate. In these situations, fleet managers should use engine hours as their maintenance guide.

Managing the Fleet

Fleet managers should be looking at engine run time hours for their maintenance schedule, especially if they have vehicles with high idle time. Normal engine hours, in combination with mileage, would be 250 engine hours within 7,500 miles. But, if a fleet manager sees 500 engine hours for just 7,500 miles on the odometer, that’s not good. Especially if they waited until after 7,500 miles to change the oil. They need to look at engine hours for maintenance intervals and not mileage, because the vehicle mileage is underreporting the actual engine wear.

Godding told the story of how a tow truck owner was frustrated because he needed an entire diesel engine overhaul after only 250,000 miles. “The customer couldn’t believe that his engine already needed to be overhauled. He thought it should last at least 500,000 miles before it needed an overhaul. I told him I would look into the issue.”

“What I found was that the number of engine “run-time hours” actually equaled 500,000 miles of travel time on the truck. The odometer wasn’t an accurate description of the wear and tear the engine went through. I asked the owner how often he was doing oil changes, and he said at every 12,000-mile service interval. But, based on his engine hours, he was actually doing an oil change about every 25,000 miles,” relates Godding. That was an expensive lesson for the tow truck owner.

Routine maintenance schedules, like changing engine oil, can drastically change when using engine run time as a guide, as opposed to mileage.

To see if fleet managers use this technique to schedule maintenance, I spoke to Bob Harrison, head of fleet services for the City of Redding, California. Bob oversees the maintenance of numerous city works vehicles, including bucket trucks, snow removal equipment, and other construction support vehicles.

Harrison said that, for years, he has been using engine run-time hours to map out his oil change schedules. “I know a lot of our city vehicles spend a fair amount of time idling. I use engine hours — around 250 to 300 hours — to determine when it’s time to change the oil and do a service,” Harrison explains.

If this is a daily-driven diesel-powered pickup that doesn’t have a snowplow attached to the front of it or stays at idle for extended periods of time, then vehicle mileage may still be a safe benchmark for vehicle maintenance.

It’s Not All About Hours

While engine run time hours are an excellent measure of engine use, they don’t tell you much about drivetrain usage. The vehicle’s odometer can still dictate scheduled maintenance for differentials or transmission services. For vehicles that don’t see lots of idle time, like a family pickup mainly used for towing a fifth-wheel trailer, mileage is still a good indicator of overall vehicle usage.

Next time you are at the dealership looking to purchase a used diesel work truck, it might be wise to look at the hour meter as well as the truck’s mileage and do a little math to really understand how much wear and tear the engine has endured. What is under the hood may be more used up than the odometer indicates.

Watching engine hours, instead of miles, enables a fleet manager to get a more accurate insight into the wear and tear on a vehicle and plan preventive maintenance accordingly. It can also help someone shopping for a used diesel-powered vehicle to make a more informed decision, and avoid high-wear pitfalls, masked by low odometer readings.

Regardless of whether you are a private diesel truck owner or the manager of a fleet of high-use work trucks, understanding the relationship between run time and mileage can help you take better care of your diesel engine, and keep it on a more strict maintenance schedule. After all, engine overhauls are costly, so you want to make sure you are getting the absolute most work possible out of your engine before needing one.