What It’s Like Driving The 2020 GMC Sierra HD AT4 - GM Authority

We recently shared with you how the 2020 GMC Sierra HD fares during heavy towing, and how a 2019 GMC Sierra AT4 Duramax performed during a rather grueling off-roading event up in Wyoming. But what about the 2020 GMC Sierra HD AT4? We took GMC’s latest dedicated off-road workhorse out on the open road and on rough terrain, and here’s what it’s like to drive it.

Before we head onto our impressions, let’s underline some interesting fun facts about GMC’s latest adventure machine. Since the AT4 variant has thus far proven successful on the standard 2019 GMC Sierra 1500 from a sales standpoint, it only makes sense that it be extended to the Sierra Heavy Duty pickups, right?

It’s an interesting niche that slots between a standard three-quarter / one-ton truck and a full-on dune-jumping machine like the Ford F-150 Raptor, which both Ram and Ford are also attempting to fill with the 2019 Ram Power Wagon and the 2020 Ford Super Duty Tremor.

GMC’s AT4 model adds just enough performance-enhancing components to take your heavy-duty truck off beaten paths without fear of damaging it. Among the enhancements are skid plates front and rear, tow hooks, a two-inch lift via Rancho Shocks, an Eaton rear locking differential and off-road specific tires. The Sierra HD AT4 also looks much meaner than a standard Sierra 2500HD or 3500HD thanks to a blacked out grille and wheels.

But what interests us the most is that while the 2020 GMC Sierra HD AT4 can be powered by GM’s tried and proven 6.6L L8T V8 good for 401 horsepower and 464 pound-feet of torque, it can also be fitted with the 6.6L L5P V8 turbo-diesel Duramax engine which pumps out 445 horsepower and 910 pound-feet of torque, allowing the 2020 GMC Sierra HD AT4 to be the dedicated off-road pickup truck that boasts the highest towing rating in the segment.

We drove the Sierra HD AT4 both on and off the road. Just like the Denali model we drove to tow that giant trailer, we were pleased by the Duramax engine’s power delivery, smooth and free of unwanted turbo lag. The Allison-branded ten-speed automatic transmission was also a gem as it always seemed to find the proper gear and downshifted smoothly when applying throttle.

The Rancho Shocks suspension upgrade was also impeccably smooth, both on the road and on the bumpy stuff during our short off-road stunt. However, the off-road tires did lead to higher-than-average cabin noise. We were also disappointed by the rattles we heard from the truck’s cabin when things got bumpy.

Except for a Spartan interior with questionable build quality in some areas, we found the GMC Sierra HD AT4 to be a convincing package for anyone desiring off-road capability without losing any of the heavy-duty benefits.

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2020 GMC Sierra HD Photos