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2020 GMC Sierra 2500 AT4 Duramax Diesel Review

Does GMC Have a Viable Off-Road Package for Their All-New Heavy-Duty Trucks?
The availability of serious off-road packages for pickup trucks has increased dramatically in the last ten years. Today, we are fortunate to have Ford Raptors, Ram Rebels, and Chevy Trail Bosses, as well as GMC AT4s. But every one of these rigs is light-duty. When it came to off-road packages on heavy-duty pickups, the Power Wagon, and to a lesser extent, Ford’s FX4 and GM’s Z71 options have been the only contenders. Well, that’s about to change. For 2020, Ford has announced its new, factory-lifted Tremor package. And GMC has developed an AT4 package for its all-new 2500 and 3500 HD trucks.

GMC had tremendous success in marketing the sub-brand Denali. These swanky street trucks now make up a third of all GMCs sold. That’s staggering. And these trucks aren’t cheap. GMC says the transaction prices for Denalis are higher than that of Jaguar, BMW, or Audi. So, it made sense for the truckmaker to expand the off-road AT4 sub-brand throughout the GMC lineup.

Now, let’s cut to the chase here. The level of the off-road capability promised by the AT4 trim isn’t the same on every GMC. And, the one developed for these heavy-duty trucks here isn’t quite as hardcore as the Power Wagon or the Tremor. But it enough to provide an improvement in the big GMC’s off-road capability? To find out, we spent a week with a $76,960 Summit White Sierra 2500 Crew Cab AT4. We put it through our usual traditional on and off-road 300-mile test loop. Let’s see if the AT4 package is worth the extra dough.

The new 2020 GMC Sierra 2500 AT4 is a competent workhorse, especially when equipped with the Duramax Diesel. Our test unit could handle a hefty 3,048 lb. payload and tow a 18,700 lb. trailer. 

The Hardware

GM completely redesigned its heavy-duty pickups for 2020. To our eyes, this traditional GMC design is the more handsome of the two. When you add the $4,215 AT4 package, it looks even better. On the outside, the AT4 wears beefy bright red tow hooks, blacked-out trim, and optional 275/65R20 Goodyear Wrangler Trailrunner A/T tires that measure about 34-inches tall. The AT4 comes with full skid plating underneath. And off-road-ready Ranchos complement the standard-height springs. The AT4 also includes a heads-up display inside with a neat digital inclinometer. And, of particular interest to us, there’s a rather amazing surround-view camera system option that provides live, low-speed trail video from just about any angle.

The new GMC HD trucks were designed to handle much larger loads than the outgoing model. And, you can tell just by looking at that enormous front grill. It’s there to provide cool air for a massive new radiator and a 28-inch diameter cooling fan assembly. Under the hood of our truck was the stout Duramax diesel. The $9,890 engine is the same L5P-spec Duramax that became available back in 2017. That means the 6.6L still generates 445 horsepower at 2,800 rpm and a substantial 910 lb-ft of torque at just 1,600 rpm. That’s 45 more horsepower than the most potent Cummins but 90 lb-ft of less torque. Ford has announced that its new 2020 Power Stroke diesel will deliver 475 hp and 1,050 lb-ft of torque. So, our GMC takes the bronze medal when it comes to torque ratings.

Perhaps the most significant improvement for those of us that like to take our diesels in the dirt is the relocation of the DEF tank. On previous GM heavy-duty trucks, that tank rode on the outside of the passenger-side frame rail just aft the rear wheel. There it was susceptible to trail damage. The new tank lives under the bed where it should.


Under the hood of this big GMC is an equally massive Duramax V8. It offers the same 445 horsepower and 910 lb-ft of torque as the last-generation. But GMC increased the rig’s cooling capacity with a giant new fan and radiator. An all-new Alison 10-speed transmission backs the big diesel. We recorded a solid 18 mpg on our 300-mile test.

The Duramax might be the same, but it’s paired to a new Alison automatic. The new 10L1000 ten-speed automatic has a much deeper 4.45:1 first gear than the old 6-speed’s 3.094:1. Second and third gears on the new Alison are lower than the second gear in the previous transmission. The lowering gearing takes advantage of the Duramax’s horsepower and torque curves better. But it also means better off-road performance.

And speaking of off-roading, the AT4 has a unique transfer case. The push-button, two-speed ‘box retains the low-range ratio, but adds an all-wheel-drive function like some light-duty models called “Auto 4WD”. That’s entirely unique for the heavy-duty market. It’s very smart because it allows someone that might be hauling a big load or towing a heavy trailer in foul weather to leave the transfer case in that all-wheel-drive mode. They can enjoy the benefits of better traction in all speed conditions as the truck transitions from wet to dry roads. There’s also a Traction Select system that provides a specific off-road mode that alters throttle progression and other electronic parameters—but only in high range.

To help handle bigger loads, GM engineers up-sized the frame so that, on the most capable models, it can tackle gross combined weight ratings up to 43,500 lbs. and tow a hulking 35,500 lb-trailer. The trucks sit on a longer wheelbase now (5-inches longer on our Crew Cab), which created additional space inside.

The rear axles are larger with the 2500-series trucks using an 11.5-inch ring gear. 3500-series trucks use one with an even heftier 12-inch gear. Our AT4 came with 3.42:1 gears and the Eaton automatic locking rear differential. The open front 9.25-inch differential is a carryover from the previous truck. The front and rear suspensions are a new design. But they continue to use leaf packs in the rear and torsion bars up front.

Despite our pleas over the years, neither end of the suspension received any additional wheel travel, according to chief engineer Tim Herrick. Some might question the logic of GM retaining the independent front suspension on a heavy-duty 4WD truck. But Herrick says a live axle was never even considered. GM has always claimed the trucks ride and handle better because of it. That’s debatable.

The HD’s beefier drivetrain components certainly increased the truck’s curb weight over previous GM heavy-duty models. The last one we tested, a 2017 Chevy 2500 HD LTZ Crew Cab diesel, weighed 7,980 lbs. This new AT4 with similar levels of equipment weighed 8,300 lbs. The 2017 truck could only handle a 1,926 lb payload. This new one? A whopping 3,048 lbs. thanks to its 11,150 GVWR. Our AT4 can also move an 18,500-lb trailer—up about 4,000 lbs. over the last-generation model. Though it weighs more, this new GMC is undoubtedly a more capable hauler.

GM equips it’s 2500 series trucks with an 11.5-inch rear axle. Despite wearing 34-inch tires, there were just 9 inches of ground clearance beneath it. And we drug the center of that axle often. Like the front, the rear suspension is damped by Rancho shocks. We found this truck’s suspension to be a rough ride both on pavement and off.

On the Street

The last-generation 2500-series Duramax diesel was a downright rocket on the street. Our friends at Car and Driver recorded a 0-60 mph time of just 6.2 seconds. And despite the weight gain, this one feels just as quick. The new 10-speed automatic is brilliant at cracking off short, rapid shifts and keeping the big V8 always in its powerband. It’s ridiculous to call a four-ton heavy-duty pickup sporty. But, well, this one certainly felt a little like that to us. It rips whether on a backcountry road or freeway slog.

However, this is a big truck. It’s not exactly well suited to sneaking around our tight Los Angeles neighborhood or dicing through traffic. Good thing then that it was loaded down with just about every safety tech option available. But the one piece of tech we might like to leave behind is the Forward Collision Warning. It’s a system designed to avoid head-on collisions by judging your distance and speed compared to the vehicle you are following. This one is a bit too sensitive and reacts aggressively by vibrating the seat and flashing dash lights too often.

Speaking of the interior, the HDs don’t break new ground here. The design is a little plain and not exactly a step forward. That’s especially true with its 8-inch infotainment screen. It works well but is dated compared to the tablet-sized screen on other trucks. Still, the cab of this workhorse is noticeably roomier than that of the outgoing trucks. And rear-seat accommodations are incredibly spacious.

GM has always touted that it’s heavy-duty trucks ride and handle better because they use an independent front suspension. While it did steer well, we had a hard time seeing any benefit when it came to ride quality. If we’re honest, the big GMC rode fairly rough. Now some of that could be attributed to its huge 3,000 lb. payload. On the freeway, the rear end of the truck bucked far harder than the last Power Wagon we tested. In fact, the Power Wagon rides better over any terrain. While the AT4 wasn’t the smoothest truck in terms of ride quality, it was very quiet. On the highway, it’s commendably silent with little noise from the engine or wind.

Locked in cruise control at 70 mph, the AT4 showed just 1,500 rpm on the tach. And, unlike most trucks we test, the GMC never needed a downshift—no matter how steep the freeway’s grade. The torque and gearing of this rig are a perfect match. Similarly, the truck’s exhaust brake and Tow/Haul mode did an excellent job slowing us down too.

Throughout our 300-mile test, this AT4 returned a solid 18 mpg. That’s 1.2 mpg worse than the 2017 model with the same motor (credit perhaps the weight gain and physically larger bodywork). But it’s still a better number than most of the other diesel heavy-duty trucks we’ve tested. Thanks to the 36-gallon fuel tank, with that fuel economy we could travel nearly 650 miles before needing fuel.

The new cabin of the 2020 GMC HD is definitely roomier than the previous generation. However, its 8-inch infotainment system is starting to look a little dated when compared to other trucks with much larger screens. 

On the Trail

GM has finally realized that 4X4 fans don’t appreciate low-hanging air dams. This AT4, as well as all heavy-duty GMCs, have a much-improved taper to those lips. In fact, the nose of the truck seems to sit physically higher than before. And because this is an AT4, there is skid plating protecting the front diff. Ground clearance up front is excellent with a full 10 inches under that protective plate. That’s an inch more than we had in the last-generation truck. But the large rear axle means there are only 9-inches before it drags.

In sheer terms of off-road package capability, this AT4 doesn’t pack nearly the same level of hardware as some. And it does ride rough on the trail. There’s quite a bit of head toss. Sharp rocky terrain taken at speed creates some quaking in the cab’s structure.

Still, on our toughest hill climb, the AT4 crested the top. On our first attempt at crawling the hill, it ground to a halt near the top. We discovered that the rear diff had plowed into the dirt and halted progress. Once we created a little more momentum, the big GMC crested the hill. The truck’s hill descent control system easily slowed the four-ton beast on the way down, and those surround cameras provide a great view. This truck has 15 unique camera views available—it’s is a system that should be standard on every 4X4.

The Goodyear Wrangler TrailRunner AT tires feature an all-terrain tread and measure out to almost 34 inches tall. Wrapped around 20-inch wheels they were fairly stiff. And, even with 34 inches of tire diameter, the massive rear axle of the GMC tended to get hung up a decent amount. 

The frame and suspension of the 2020 GMC heavy-duty are all new. But the design is similar to previous generations. And up front, that means an independent torsion bar front suspension. The AT4 package includes lots of useful skid plating and Rancho dampers.

Predictably deep sand is not this truck’s friend. The stiff sidewalls of the tires combined with the stiff springs made tackling our usual sandy wash impossible in 2WD. The truck’s big torque right off idle gets the tires spinning and the rear axle hopping. It quickly digs down instead of forward. Auto 4WD, as well as 4WD high range, both help get the truck moving. However, the suspension is so stiff that the whole truck vibrates and shakes as you pour on power and speed. So, after just a few minutes playing in the sand, we moved on. Unfortunately, the truck’s performance on the park’s fire roads was equally poor. The ride is bone-jarring, even at mellow speeds. So, we made our way to the dirt mogul section.

Here, the truck did have adequate clearance and traction to complete the climb. But it wasn’t exactly elegant. The rear locker requires wheel speed to engage. There’s plenty of dust before the truck eventually lurches a few feet up the trail. The tire spinning begins again and then another lurch and lots of dust. The AT4 did touch down on its side steps once, so we adjusted for a slightly less aggressive line. And, because the rig has limited suspension articulation, the rear tires spent quite a bit of time in the air.

The AT4 is not a serious off-road package. The suspension ran out of wheel travel quickly, the ride is stiff for maximizing its GVWR, and those tires weren’t very aggressive. Still, it was able to go places we hadn’t been able to in the last-generation GM heavy-duty trucks.

The Bottom Line

This truck is all about the powertrain and its load-carrying capability. The Duramax is very potent. When paired to the new 10-speed, it’s downright excellent at everything it does. Because the new chassis can handle so much more payload and towing, it’s a far stouter workhorse. It’s a machine built for covering lots of miles with lots of weight behind it delivering excellent fuel economy.

As an off-road package, the AT4 equipment just isn’t impressive. Sure, this truck will go more places than the last one we tested. But so would a 2500 HD without the AT4 package. When compared to a more serious machine like the Power Wagon, this one would be left in the dust. Our advice? If you like the look and are more interested in hauling than four-wheeling, pick one up. But if we owned an AT4, there would be plenty of upgrades on our list before we hit the trail again.