When you’re hauling a 3,000-pound truck camper, an engine doesn’t get much opportunity to hide its shortcomings. Mountain grades expose a lack of power, long descents test the braking system, and thousands of miles carrying a heavy load quickly reveal weaknesses in reliability and fuel economy.
And after 110,000 miles behind the wheel of a 2013 Ram 3500, we’ve had plenty of time to find out what the 6.7L Cummins Turbo Diesel does well—and what we don’t like about it.
Our truck is equipped with the 68RFE six-speed automatic transmission and 3.73 axle gears, with the Cummins producing 370 horsepower and 800 lb-ft of torque. For much of those 110,000 miles, the truck’s primary job has been hauling a 3,100-pound truck camper, including plenty of highway miles and mountain driving.
Overall, the Cummins has been outstanding. Its tremendous low-rpm torque makes climbing mountain grades almost effortless, its exhaust brake is one of the best features we’ve ever had on a truck camper rig, and we’ve averaged an impressive 12 mpg with the camper aboard.
It hasn’t been perfect. We’ve had a DEF pump failure that put the truck into the dreaded limp mode, a cracked plastic radiator pipe, and we’ve grown thoroughly tired of adding DEF about every 3,000 miles.
But after 110,000 miles, those complaints make up a surprisingly short list.
The 6.7L Cummins by the Numbers
The Cummins displaces 6.7 liters, or 408 cubic inches, and unlike the V8 diesels offered by Ford and GM, uses a traditional inline-six configuration. The engine employs a cast-iron block and cylinder head, 24 valves, high-pressure common-rail fuel injection, turbocharging, and intercooling.
Bore and stroke measure 4.21 and 4.88 inches, respectively, with a 16.5:1 compression ratio. In our 68RFE-equipped truck, the Cummins produces 370 horsepower at 2,800 rpm and 800 lb-ft of torque at just 1,600 rpm.
The engine holds approximately 12 quarts of oil with the filter, while the cooling system holds nearly 30 quarts.
Backing the Cummins is the 68RFE six-speed automatic transmission. Our truck also has 3.73 axle gears, which we’ve found to be a good compromise between pulling power and relaxed highway cruising.
Those specifications are impressive enough, but numbers don’t tell you what it’s like to live with an engine for 110,000 miles while carrying a ton and a half of camper.
That’s where the Cummins really shines.
Torque, Torque, and More Torque
The first thing you appreciate about the Cummins with a heavy truck camper aboard is the torque.
Lots of engines produce impressive horsepower. What matters more when 3,000 pounds of camper are sitting in the bed is how easily the engine can move all that weight, particularly when the highway starts climbing.
This is where the Cummins earns its keep.
Point the Ram toward a mountain pass, roll into the throttle, and the big inline-six simply digs in and pulls. There’s little drama and none of the feeling that you’re asking the engine to do something it would rather not do.
Having 800 lb-ft available at only 1,600 rpm is a big reason why. The engine doesn’t need to scream its way up a mountain. It develops tremendous pulling power relatively low in the rev range.
With a 3,000-pound camper aboard, the Cummins has had no trouble maintaining speed on long mountain grades. Even on extended climbs, the engine feels remarkably relaxed and well suited for the job.
If there’s one word that describes this engine under load, it’s effortless.
The Down slope Is Even Better
As impressive as the Cummins is climbing mountains, one of our favorite features becomes apparent after reaching the summit.
The factory Smart Exhaust Brake is superb.
Think of it as an exhaust brake on steroids. The system uses the variable-geometry turbocharger to create backpressure and slow the truck, but the real magic is in the electronic controls that determine how aggressively the system responds.
Operation is simple. A button on the lower switch panel of the center stack allows the driver to select between the normal exhaust-brake setting and the more aggressive automatic mode. In the standard mode, the exhaust brake provides strong, predictable braking whenever you lift off the accelerator. The automatic mode takes things a step further, varying the braking effort to help maintain the desired vehicle speed on a descent.
In practice, we rarely needed the more aggressive setting. The standard exhaust-brake mode was more than sufficient for our needs, even with a 3,000-pound camper aboard.
And that’s saying something.
Engage the exhaust brake before starting down a steep mountain grade and the Cummins provides an impressive amount of braking force. Rather than constantly riding the service brakes to keep the truck and camper under control, the exhaust brake handles much of the work.
We’ve descended some pretty serious mountain grades with very little need to touch the brake pedal.
That’s more than a convenience. It dramatically reduces heat and wear on the truck’s service brakes, keeping them cool and ready when they’re actually needed. With several tons of truck and camper heading downhill, that’s a feature we’ve come to appreciate immensely.
For truck camper owners who spend a lot of time traveling through mountainous country, we’d put the Cummins exhaust brake near the top of the engine’s best features. The torque may get you to the summit, but the exhaust brake is what makes the trip down the other side so much more relaxed and controlled.
Fuel Economy
Then there’s fuel economy.
Our camper weighs approximately 3,000 pounds, so this Ram hasn’t spent its life running around empty. In addition to the weight, a full-size truck camper presents an enormous frontal area and plenty of aerodynamic drag.
Yet we’ve averaged approximately 12 mpg while carrying the camper.
That’s pretty impressive for a heavy-duty truck hauling what amounts to a small house through the air.
Of course, mileage varies considerably depending on highway speed, elevation, headwinds, terrain, and driving style. But after 110,000 miles with this truck, we’ve been very pleased with the balance between pulling power and fuel consumption.
The 3.73 axle ratio has also been well suited to the way we use the truck. There’s plenty of pulling power when we need it without saddling the truck with extremely deep gearing for long highway trips.
Reliability After 110,000 Miles
After 110,000 miles, reliability is probably one of the strongest arguments we can make for the 6.7L Cummins.
The Cummins engine itself has been excellent. Considering the amount of time our Ram has spent carrying a 3,100-pound camper—including plenty of mountain driving—we’ve been impressed by how little trouble the powertrain has given us.
We’ve experienced only two noteworthy problems.
The first, and by far the most aggravating, was a failed DEF pump. When the pump failed, the truck eventually went into limp mode, severely restricting performance until the problem could be repaired.
That’s particularly frustrating because the Cummins itself can be running perfectly well while an emissions-system component effectively sidelines the entire truck.
Our other noteworthy issue was a cracked plastic radiator pipe.
Neither problem involved an internal failure of the Cummins itself. Considering the mileage, the loads we’ve carried, and the places we’ve traveled, that’s a pretty good track record.
The DEF System: Our Biggest Complaint
As much as we like the 6.7L Cummins, there’s one aspect of owning this diesel that we could definitely live without: Diesel Exhaust Fluid.
Our 2013 Ram 3500 uses an SCR emissions system that requires DEF, and in our experience we’re adding fluid roughly every 3,000 miles.
It’s not particularly difficult or expensive, but it gets old.
On a long truck camper trip, we’re already thinking about diesel fuel, fresh water, propane, battery power, campsites, and everything else involved with keeping a truck camper supplied. Having another fluid level to monitor and another jug to buy and pour into the truck is simply an annoyance.
After 110,000 miles, we’ve purchased more than enough DEF to know that we’d be perfectly happy never seeing another jug of the stuff.
The fact that we’ve also experienced an actual DEF pump failure makes our feelings about the system even stronger. In that instance, a component that had nothing to do with the Cummins’ basic ability to run and produce power put the truck into limp mode.
That’s one of the realities of owning a modern diesel. The core engine can be exceptionally robust, but the emissions equipment surrounding it introduces additional complexity and potential failure points.
The Diesel Weight Penalty
There’s another downside to the Cummins that’s especially important for truck camper owners: weight.
The Cummins engine and its associated diesel hardware are heavy—about 800s in all—and that additional weight counts against the truck’s available payload capacity before a camper is ever loaded.
That’s something every truck camper buyer should consider.
A gasoline-powered heavy-duty truck can offer a meaningful payload advantage simply because its powertrain weighs less. If maximizing payload is the overriding concern, a gas engine deserves serious consideration.
But there’s another side to that equation.
When you’re routinely crossing mountain ranges carrying a heavy camper, the Cummins makes a compelling argument for itself. You pay a weight penalty for the diesel, but what you get in return is 800 lb-ft of low-rpm torque, excellent fuel economy under load, and an outstanding exhaust brake.
For the way we’ve used this truck, that’s been a trade we’re happy to make.
Would We Choose the Cummins Again?
Absolutely.
Our 2013 Ram 3500’s 6.7L Cummins, 68RFE six-speed automatic, and 3.73 axle ratio have proven to be an excellent combination for hauling a heavy truck camper.
The Cummins produces more than enough torque to pull our 3,000-pound camper up long mountain grades without breaking a sweat. The exhaust brake provides outstanding control coming back down. And averaging around 12 mpg with the camper aboard is impressive considering the weight and aerodynamic drag involved.
Reliability has also been excellent. Aside from the failed DEF pump and cracked plastic radiator pipe, we’ve had little to complain about mechanically.
Well, except for buying DEF every 3,000 miles. We could definitely do without that.
But perhaps the thing we appreciate most about the Cummins isn’t something you’ll find on a specification sheet.
It’s how relaxed the engine makes the truck feel.
Load the camper, point the Ram toward the mountains, and the engine rarely seems bothered by what’s sitting in the bed. There’s plenty of torque waiting when the grade gets steeper, and when you finally reach the summit, the exhaust brake takes over for the trip down the other side.
After 110,000 miles, countless mountain passes, and thousands of miles carrying a 3,000-pound truck camper, we’ve gotten to know this engine pretty well. The 6.7L Cummins isn’t perfect. It’s heavy, its emissions system adds complexity, and we’re thoroughly tired of buying DEF every 3,000 miles.
But those complaints are overshadowed by what the engine does exceptionally well: tremendous low-rpm torque, outstanding downhill control, excellent fuel economy under load, and impressive long-term reliability.
After 110,000 miles, we’d choose the Cummins again.
With a Host Yukon there’s nothing I’d rather drive than a diesel when we’re traveling around in the Rockies. We usually plan a truck stop when we’re getting down to 1,000 miles or so of def remaining, and a Buc-ee’s is even easier since we can get a BBQ sandwich too! I also run both a diesel and def additive too, which adds another logistics issue to consider but Amazon lockers are everywhere now or I can just bring along extra, it’s cheap insurance. After 120k miles in 3 years we’re pretty used to either putting stuff in tanks or emptying them out.
There’s also the entire electrical system to consider with 500 ah of lithium batteries, 600w solar, 30A DC-DC and plug ins…which I find myself considering much more than def, especially with the summer we just had. Keeping us, the fridge and the batteries cool was (def)initely much more consuming! 😂