“This truck is capable of towing 35,500 pounds. That’s about half of what a semi truck can haul,” emphasized GMC Executive Chief Engineer Tim Herrick. He was referring to the top-spec 2020 GMC Sierra HD 3500 dually, with a 6.6-liter Duramax turbodiesel V8 and Allison 10-speed. Even without the headline-grabbing 1,000 lb-ft of Ram’s Cummins engine, the Sierra manages to pull a few more pounds, largely thanks to optimized gearing.
(Full Disclosure: GMC flew me to Jackson Hole, Wyoming, to drive the new GMC range of trucks, put me up in a nicely equipped tiny house, and put a lot of great food in my stomach.)
As with any full-sized heavy-duty pickup, the first thing you notice about the Sierra HD is how large this mother is. I’m 6-foot 2-inches with a 32-inch inseam and it’s a serious chore to haul myself up into the driver’s seat of this vehicle. That’s not in my imagination; the 2020 HD is taller and longer than the truck it replaces.
In a sense, a truck like this is something like a track toy sports car. It has limited usefulness, it’s not great at everything, but it is exceptional at the task it was built for. If you frequently need to tow or haul thousands of pounds, this workhorse will make it feel effortless, both through brute strength and technology. If you’re the kind of person that absolutely must tow an entire house with you when you go “camping,” then this is the kind of truck you’ll need to pull it.
GMC says that 93 percent of its Heavy Duty customers use their trucks to tow, and 50 percent tow at least 8,000 pounds on a regular basis. Apparently that usually means a fifth-wheel camper; the kind that swivels on a huge hitch (the fifth wheel) in the truck’s bed.
Specs That Matter
GMC claims that this truck is entirely new, with no carryover parts from the previous HD truck. The 2020 version is more powerful, whether gasoline or diesel. It is also bigger, heavier, and can tow a whole lot more.
The 10-speed transmission is a larger and more robust extrapolation of the car-based GM/Ford 10-speed auto. Instead of working with Ford to develop this one, GMC called up Allison Transmissions to help engineer the big gearbox to a point that it would put its stamp of approval on it.
You wouldn’t think a 10-speed gearbox would be necessary in a truck with a giant diesel V8, but it was almost unbelievably smooth while I had it going to work. The truck always seemed to be in the right gear, no matter the situation. It was a nice change from the old three-speed in the pickup I drove in college.
The new truck has a raft of improvements from the one it replaces, including a new Head-Up Display unit, camera display in the rearview mirror, electronically selectable automatic 4WD, and of course the new Multi-Pro tailgate. Plus, all the camera angles you could ask for.
That tailgate can now also be optioned with Kicker-branded Bluetooth speakers integrated, which I really like. It’s also got much more headroom and front-seat legroom than before. The back seats are still big enough in the Crew Cab that I can sit behind another tall person with no issue.
If you’re looking for radar cruise control from your near-80-grand pickup, however, you won’t find it. GMC is still working on rolling out adaptive cruise to the Heavy Duty lineup, but hasn’t quite made that happen yet. That caught me off guard the first time I set the cruise and rapidly closed on the car in front of me. GM has a good radar cruise system, and it should be using it in this truck. It’s borderline inexcusable.
Aside from perhaps reversing, the worst part of towing a trailer is hooking the damned thing up to the truck. It’s traditionally a two-person job, as you’ve got to have somebody standing at the trailer directing the driver where to back up and how far. Then, once you’re lined up, you put the truck in park and it rocks against the parking pawl, putting you out of reach of the hitch again. Then you have to check tire pressures, make sure the lights are all functioning, and (if you’re going camping) check the water tanks and propane.
Thanks to some pretty nifty tech, GMC consolidates these tasks. To start, the myriad camera views ensure you’ll back your truck’s tow hitch directly underneath the trailer hookup straight off. Then, as soon as you move into park, the truck automatically clamps on all four brake calipers to guarantee the tow ball stays put. Once you’ve got the trailer lights all hooked up, you can put the truck in a light testing mode, where it will cycle the brake lights and turn signals while you go around back of the trailer to make sure they’re lighting up properly.
If you tick the right boxes on your truck and trailer, you can even check your fresh and grey water tank levels, propane tank levels, and trailer tire pressures from the infotainment screen inside the truck. You can also have your trailer hard-wired with an interior camera so you can check on your cargo load while driving, or make sure your camper hasn’t been invaded by a family of raccoons while it was in storage.
If you’re a hardcore camper, you know how important it is to have air conditioning in the depths of America’s great wide open wonder. Because of that, GMC made sure you could kick on your AC unit 20 minutes or so before you arrive at your camp spot. Just poke at the touch screen in the middle of the dash for a second and your house on wheels can start cooling off while you’re still on the road. You wouldn’t want to be mildly inconvenienced with a warm trailer when you get to your destination, right?
Tow With The Flow
An updated independent front suspension setup is said to make the truck more stable and easier to drive when loaded down or hauling, compared trucks with solid front axles like the Ram HD and Ford Super Duty. I believe the hype, because I was immediately pushed into the 3500 HD model with a 14,000 pound trailer hooked on the back. It hauled that trailer like it was barely even there, and the truck’s comfort and stability didn’t seem impacted at all.
For what it’s worth though, a solid axle is generally considered stronger, which is why it hasn’t been abandoned by the rest of the industry.
The route for our test tow job was relatively simple, negotiating a half-dozen straight Wyoming highways with a couple of 90-degree turns mixed in. The roads were plenty wide for a normal car, but this truck and trailer combo took up the whole lane and then some. I never felt like I would get myself into something tight on this route, but there were certainly a number of side streets that I absolutely would not have wanted to drive down. Maybe skip canyon carving if you’re driving one of these. (Or, use it to tow a sports car or three!)
The trailer itself, in case you were wondering, is a Keystone Alpine 3700FL - Fifth Wheel. It features a pair of air conditioning units, automated hydraulic leveling, five slide-outs, a shower, a washer and dryer, a full kitchen, and an entertainment room with theater seating. It can sleep at least four. And the MSRP on the one I hauled was $93,279.
Did I mention it’s really big?
The 6.6-liter turbo diesel pulled it like an industrial implement. It was built for this job, and it was pretty good at it. This truck is said to be capable of a 19.9 second 0-60 time while dragging an 18,000 pound trailer. That’s not exactly blazing speed in a world where minivans can run five-second sprints now, but come on, the engine’s moving north of 23,000 pounds, not counting your fat butt in the driver’s seat.
Power-extending tow mirrors are a nice touch. But remember to retract them when you aren’t hauling anything, for fear of looking like a dork.
And braking? Yeah, it’s got that. The truck’s stock disc brakes are strong enough to haul the whole thing down quickly and without drama. If you want to save temperatures on your rotors, however, there’s an exhaust brake button on the diesel models which helps keep speeds in check on downhill slopes on the highway.
The variable-geometry turbocharger will intentionally create an increased pressure in the exhaust manifold to slow the truck down. And the truck comes with an integrated trailer braking system to help keep the heavy-as-hell tail from wagging the dog.
What’s the downside of this, then? While hauling this pied-sur-roues, I saw real-world fuel economy in the mid-to-high single digits. Call it eight-ish miles per gallon. Whatever it is, it’s not good. Especially with the cost of diesel these days.
The current national average price for diesel is $2.93 per gallon. And if you’re paying about 37 cents to drive a single mile, it’s going to cost you about $366 to go camping 500 miles away. From where I live in Reno, it would cost almost twice as much in fuel to drive to Salt Lake City as it would to buy a plane ticket. Then again, if you have an $80,000 truck and a $90,000 camp trailer, what’s a few hundred more in diesel?
Heavy Duty Driving
GMC put me in so many different trucks throughout this program: A 3500 HD hauling a fifth-wheel camper trailer, a 2500 HD bumper-pulling an enclosed car hauler, a diesel AT4 bouncing around an off-road course, a gasoline AT4 driving around dirt farm trails with a bed full of logs, and a 3500 HD dual-wheel hauling an open trailer with close to the truck’s max capacity loaded down in ballast. They all share a lot, obviously, but there’s some differentiation.
I really like the Heads-Up Display and the camera-display rearview mirror. The rest of the camera views are selectable through the center console screen, which is great, but the screen itself is lackluster. Perhaps it’s the behemoth quality of the truck’s interior that makes the screen seem tiny, but it is truly dwarfed by the rest of the dashboard. It uses GM’s familiar user interface, which is fine, but not great.
Credit where due, I always cry out for more hard buttons and knobs, and GMC is happy to deliver. The center stack houses 25 buttons, five knobs, a trailer brake controller, USB ports, and a 12-volt outlet. A further eight buttons and two knobs are found to the left of the steering wheel to control driving modes. That all sounds great, but visually it’s a bit of a mess. Maybe once you’re used to the layout it’s a little better, but I found myself hunting for the right button more than once.
Off-Roading The AT4
AT4 is a new trim line for GMC, and offers a rugged and tough exterior appearance to foil the chrome-dipped Denali trim. The AT4 trim replaces the previous generation’s “All Terrain” trim and accentuates what was successful in that truck. You get chunkier tires, and all of the chrome is blacked out. If you order a Sierra 1500 AT4, you’ll also get a 2-inch lift. The 2500 HD models already come with a 2-inch lift, so the AT4 package remains at standard HD ride height. This particular truck was a Sierra 1500 AT4 with the 3-liter inline-6 diesel. It seemed to work fine on this course, which was designed for it to work fine.
[This post was updated to clarify the ride height of a 1500 AT4 and a 2500 HD AT4, as well as clarify which model was used to complete the off-road course.]
From the driver’s seat, the rock climb (above) looked nearly vertical, but I’d seen trucks driving up it all afternoon, so I knew it was possible. Hit the throttle and roll up the thing, it’s just that easy. This not only served to show off the truck’s approach, departure, and breakover, the right-seat-engineer had me test out the truck’s hill-hold feature, and the hill-descent-control feature. Both worked fine.
As a dedicated off-roader, however, this truck is way too big. The hood height means your view becomes all sky much earlier than I’m comfortable with. The length of the wheelbase means a pretty poor breakover angle. The steering doesn’t exactly inspire confidence in where the wheels are placed. It’s fine for soft roading, but I wouldn’t be comfortable taking it crawling.
Then I got to splash through the mud and roll over a few logs for the fun of it. I never feared the truck getting stuck. It wasn’t a difficult course, but proved the Sierra can muscle its way through some sticky stuff. The truck does look better dirty, anyway.
If you’re looking for an AT4 truck, but don’t want something as gargantuan as a Sierra, you’re in luck. One of the trucks I drove was hauling a car trailer with a lot of ballast on board, and when I turned on the camera inside the trailer I was treated to this surprise. Neat.
As a tool, the GMC Sierra HD does its designated task of heavy work quite well. If you need to haul literal tons of weight around, it does that without complaint or duress. Hauling a big load can be stressful, but this truck takes much of that stress away. If you need this particular arrow in your quiver, it’ll probably hit the bull’s eye.
If you aren’t hauling stuff 100 percent of the time, however, this truck can get old quickly. The interior isn’t a particularly nice place to spend time. The full-sized American sedan has been replaced by the full-sized American pickup, and it’s not a fair replacement. If you’re looking for a comfortable ride and a nicely appointed interior for daily driving, look elsewhere. Make no mistake, this is a workhorse with a Denali-branded leather saddle.
Everything under the skin is excellent. The diesel engine is more than capable of swallowing everything you throw at it. The 10-speed transmission is a quality piece with a smooth and refined demeanor not often found in a Heavy Duty application. The chassis retains its poise, no matter how much weight you toss on the back. The Sierra HD excels at being a truck.
Visually, I think it might be trying a little too hard to look hard. The engine intake nostrils extending up into the hood was described by GMC as being “sporty and elegant” but it looks tacked on to me. The grille is approximately 37 stories tall, and if you took the GMC badge off of it, it could belong to any brand. The geometric wheel openings look chunky and oddly shaped. I understand that there are some who would think this all looks cool, but it ain’t me.