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2020 GMC Sierra 2500 HD AT4 Duramax: Off-Road Torture Test

Here’s the Truck Your Heavy-Duty Wishes It Was.

The 2020 GMC Sierra 2500 HD AT4 combined an off-road package with an already ironclad heavy-duty platform, and we made sure to flog the truck on the dirt and the tarmac. Read on to see how the HD fared on- and off-road in the 2020 Pickup Truck of the Year test.

What's Hot

  • "Irresponsible" amount of power on tap
  • Autotrac transfer case = auto four-wheel drive
  • 6L Duramax, 10-speed transmission, red towhooks

What's Not

  • Say it with us: "selectable locker!"
  • Overbearing traction control fairies

2020 GMC Sierra 2500 HD AT4 New Features?

Although similar to its LTZ-badged cousin, the GMC Sierra 2500 HD AT4 brings a unique off-road package to the heavy-duty segment. There's nothing new about the 6.6L Duramax LP5 turbodiesel and its output of 445 horses and 910 lb-ft of torque. The news comes downstream of the engine with the Allison-branded 10L1000 10-speed automatic transmission. As described by the engineers, the new gearbox is built to withstand more power and torque than the engine in front of it can produce, and it gets it all to the ground effectively. Also new for the heavy-duty segment is the Autotrac transfer case, which allows the driver to, at the push of a button, select between 2WD, 4-Hi, 4-Lo, or Auto. While in Auto, the MP3025 transfer case will automatically transfer between 0 and 50 percent of torque to the front differential, which is said to be ideal for many low-traction driving conditions. Along with bigger driveshafts and axle ratios limited to 3.42:1, the HD AT4 comes outfitted with a quartet of custom-tuned Rancho monotube gas shocks and a set of progressive-rate rear springs to better handle off-road terrain.

Can the Sierra 2500 AT4 Flex?

The GMC Sierra 2500 HD AT4 led the heavy-duty trucks on the 22-degree RTI ramp with a score of 374.

How Fast Does It Go?

Only a fraction of a second behind its LTZ-badged competitor, the Sierra 2500 AT4 sailed down the strip gaining 60 mph from a standstill in 7 seconds flat. It took the Sierra 2500 AT4 only 15.4 seconds to cover the quarter mile while reaching 91.7 mph, a time and speed combo that led the entire lineup of pickup trucks. Although the brakes and curb weights were identical between the two, the HD AT4 ground to a halt from 60 mph in 146.4 feet, exactly 36 inches sooner than the Silverado 2500 LTZ.

Inside and Outside the GMC Sierra 2500 HD AT4

"It has that Stormtrooper look." That's what one judge penned, referring to the GMC Sierra 2500 HD AT4's body-color front bumper and blacked-out grille, and there were many more positive remarks to the overall appearance of the truck. We liked the improvements in stance given by the 4.8-inch-longer wheelbase and the extra inch of track width in the rear, though the tires left a bit to be desired, as they were milder for the all-terrain category. Having heard of its resistance to baseball bat impacts, judges were seen poring over the CarbonPro bed, wondering how it would hold up to loads of fill dirt, campfire logs, and antiquated 4x4 parts. One judge questioned the MultiPro tailgate: "How'll all these pivot points do after a few summers of loading gravel and mulch?"

When inside the HD AT4, judges commended the low level of noise in the cabin and praised the dual-zone HVAC controls with their respective "finger-friendly" knobs. One judge wrote "I guess this is necessary now" when addressing the newfound utility of the head-up display. Comments on the interior in general ranged from, "Geez, this needs a refresh," to, "It's clean, smooth, and has everything I want, where I want it." We certainly appreciated the wealth of camera angles available on the 8.0-inch IntelliLink infotainment system, which, although we didn't test this specific feature, can "see through" trailers in tow behind the vehicle.

Highway Performance

"This has an irresponsible amount of power!" came out of one judge's lips after completing the acceleration test in the HD AT4, and we can assume they were amazed by how the 6.6L Duramax and Allison 10-speed funneled the 445 horsepower and 910 lb-ft of torque straight to the tarmac. When testing the truck's response to unexpected lane departure, judges were alerted of their wandering through a vibrating seat, which some found a bit intrusive. We were never sad to sit in the cabin for extended highway pushes, and when we discussed road trips in the HD AT4, judges either nodded in approval, "I'd haul cross-country with it," or griped, "That seat is a bit too stiff for my liking."

Off-Road with the GMC Sierra 2500 HD AT4

"Most fun I've had in a 3/4-ton!" scrawled a judge after a few laps around our sand dune test track with the GMC Sierra 2500 HD AT4. Judges felt the HD AT4 "handled itself with composure" when hitting dips and bumps in the road at higher speed, without uncontrolled bouncing or excessive head toss. In fact, over smaller, washboard-sized irregularities, judges described the ride as "jarring," "rough," and "I wish paying for an off-road package made this kind of terrain feel better." Regarding the AT4 package, some judges were left wanting more, citing a lack of skidplates and a tire with more off-road attitude. When we aimed the HD AT4 up our snowed-over muddy hillclimb, we were left disappointed by how much the traction control intervened when we dug into the throttle. Attacking the hill at slower speeds also had us questioning the G80 automatic locker, which, as described by a judge, "required too much wheel speed to even be functional." Another judge quipped, "I'd vote leather seats off the island if I could have a pair of selectable lockers." The camera system in the truck worked just as well off-road as it did in tight parking lots, and as one judge wrote, "This should save my passengers from spotting duties!" We were also fans of the accessible towhooks, which were coated in red and ready to yank less fortunate vehicles out of a sticky situation.

Our Take

You'll be genuinely surprised with how well this HD handles the rougher roads between you and your destination, and it can look sharp while doing it. You might not appreciate the operation of the automatic rear locking differential or the less than stellar all-terrain tires. However, mash the pedal, and the Duramax will brighten your day.

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