12/24/19 UPDATE: This review has been updated with test results for a four-wheel-drive crew-cab Sierra HD 2500 Denali diesel model.

That GMC expects more than half of its redesigned 2020 Sierra HD pickups to be sold as top-level crew-cab Denali models tells you something: Most GMC buyers prefer their heavy-duty trucks gussied up and loaded to the gunwales with luxury and technology. While the choice between the Sierra HD and its mechanical near twin, the equally fresh 2020 Chevrolet Silverado HD 2500 and 3500, largely remains a matter of brand preference, newly available features exclusive to the GMC are finally starting to give it the feel of a premium big rig.

Difference in the Details

Make no mistake: Our initial drive in the new Sierra HD in Wyoming made for pretty much the same experience we had with the updated Silverado HD in Oregon. Both trucks are significantly bigger and bolder than before, with sturdier frames, massively increased towing and hauling capabilities, and smarter features all around. Except for its roof, none of the heavy-duty Sierra's chunky body panels are shared with its light-duty brethren, and the end result, at least to our eyes, is an imposing yet distinctly more refined and cohesive appearance than its Chevrolet counterpart's.

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HIGHS: Massive capabilities, excellent Allison transmission with optional diesel engine, helpful trailering cameras and connectivity

GMC says that feedback from HD customers wanting a more rugged look without the Denali's acres of chrome was a big reason for introducing the new AT4 model, which sits just below the Denali in the lineup and features monochromatic styling with darkened exterior trim and prominent red front tow hooks. Although the Sierra HD AT4 doesn't sport the two-inch suspension lift and knobby off-road tires as does the light-duty Sierra 1500 AT4, it does come standard with 18-inch wheels and all-terrain tires (20s are available), skid plates, and an off-road suspension with upgraded Rancho dampers that are optional on other Sierra HDs. Also included on the AT4 is hill descent control, a traction-management system with Off-Road mode, a configurable 15-inch head-up display, and a low-speed surround-view camera system.

While GMC has no plans to expand the Sierra 1500's optional lightweight CarbonPro cargo bed to the big trucks, the brand's trick, six-way MultiPro tailgate is available across the HD lineup and comes standard on SLT, AT4, and Denali models. Adding to the MultiPro's versatility is an optional dealer-installed Kicker audio system with Bluetooth, USB, and auxiliary connectivity, which brings plenty of clear sound to the outdoors without the need to leave the truck unsecured with the windows down.

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A Refined Mountain Mover

We haven't yet had the opportunity to spend time with the Sierra HD's standard 401-hp 6.6-liter gas V-8. Our drive time both in Wyoming and back home in Michigan instead has focused on the impressive power of the optional turbocharged 6.6-liter Duramax diesel V-8 that GMC expects most HD customers to choose. Although not nearly as quiet in operation as the Sierra 1500's optional Duramax 3.0-liter inline-six turbo-diesel, the HD's $9750 upgrade engine is suitably refined and brings seemingly endless amounts of grunt—445 horsepower and 910 lb-ft of torque, to be exact—thanks in part to its pairing with a new Allison 10-speed automatic transmission. The shift quality and smoothness of the new gearbox are impressively smooth, even in its Tow/Haul mode and when tugging seriously heavy loads, and it smartly downshifts in response to driving conditions to maintain a steady speed down steep grades. Toggling the exhaust brake on the center stack further lessens the stress on the Sierra's brakes.

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LOWS: Simply massive on the road, quickly gets expensive, interior should feel more upscale.

At the track, our 8260-pound test truck, a crew-cab all-wheel-drive 2500 HD Denali model with the Duramax diesel, ran to 60 mph in an impressive 6.5 seconds and covered the quarter-mile in 15.0 seconds flat at 92 mph. That pace is about spot-on with that of a similar 2017 model Sierra HD we previously tested, although the new truck did weigh an additional 260 pounds, and it's quick enough to put the big GMC ahead of the last, comparable Ford F-250 we evaluated by a few tenths in both measurements. We have yet to test a similar configuration of the latest Ram HD pickup with its optional Cummins diesel inline-six, some versions of which develop 1000 lb-ft of torque, but the Ram models we have strapped our test gear to cannot keep up with the diesel Sierra HD 2500. If there is such a thing as a hot rod heavy-duty pickup, the new GMC and Chevy HD rigs are it.

Fuel economy is not quite as impressive, though. The EPA does not rate heavy-duty pickups, but our unladen test truck averaged a mere 12 mpg while in our care and posted 17 mpg on our 75-mph highway fuel-economy test, both slightly behind what we got from the previous-gen truck.

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Whether empty or heavily laden, the Sierra HD rides commendably well for a heavy-duty truck over most road surfaces, soaking up bumps and undulations with decent compliance and little chatter from the rear axle. The Sierra and Silverado HDs remain the only heavy-duty pickups in the segment with independent front suspensions, and while we wouldn't call them agile, these behemoths are fairly easy to maneuver and responsive to steering commands. The lateral grip of our Denali test truck with 20-inch wheels and LT275/65R-20 Goodyear Wrangler Trail Runner A/T all-terrain tires topped out at 0.73 g on the skidpad, and the truck needed 207 feet to stop from 70 mph, which is about what you'd expect from more than four tons of pickup.

All of the new GMC HD trucks we've driven on the street have been all-wheel-drive crew-cab diesel models, including single-rear-wheel 2500 AT4s and Denalis, both empty and loaded with about 2000 pounds of logs in the bed, which seemed to make little difference to the GMC's ability to stop and go. (Maximum payload capacity for the new Sierra HD is 3615 pounds.) As with the new Silverado HD, perhaps one of the biggest things to acclimate to in the Sierra HD is its increased ride height and seating position, which, combined with their general refinement, greatly diminishes the sense of speed reaching the driver. These are simply massive vehicles that stand out in a parking lot of crossovers like a medium-duty dump truck.

View Photos Michael Simari Car and Driver

Aided by the effective tuning of the 10-speed Allison gearbox, the Duramax has plenty in reserve when pulling a large trailer. Even in a dual-rear-wheel 3500 model hitched to a roughly 12,000-pound fifth-wheel camper, pulling out into traffic or accelerating around slower vehicles is surprisingly uneventful. You definitely feel the trailer's weight, but it's easily controlled and never gives the sense that it's "driving the truck." While dragging a gooseneck construction trailer loaded to the Sierra HD 3500's class-leading maximum of 35,500 pounds will significantly tax its performance, the big GMC sits level and still will move out with determination.

Towing a 14,000-pound conventional box trailer behind a single-rear-wheel 2500 model showcased one of the Sierra HD's coolest features, the optional ProGrade Trailering system, which employs six cameras on the truck and two optional cameras for the trailer to provide up to 15 views around the vehicle and of what's behind it through the center touchscreen display in the dash. Even for seasoned towers, the ability to see around, in, and virtually through it—with the trick transparent trailer view—brings precious reassurance and confidence. Further convenience comes from the Sierra HD's ability to store different trailer profiles for tracking maintenance; to interface with and to control the functions of certain trailers with the optional In-Command controller and myGMC smartphone app; and the ease with which the truck's computers and cameras make it for just one person to hitch up a trailer and check that its lights are functioning.

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Room to Grow

As with the light-duty GMC Sierra, the HD models share much of their interiors, save for some trim and small details, with their Chevrolet Silverado counterparts. The crew-cab offices of the up-level trucks we've driven offer cavernous accommodations front and rear, straightforward ergonomics, and intuitive infotainment and driver information systems with loads of connectivity options.

Yet, in such a tony, high-tech rig, the GMC's cabin materials and design are underwhelming, coming off as only a modest improvement over the previous model's and an almost negligible one over the latest Chevy's. The Denali's few and unimpressive pieces of unique trim and wood finish smack of a particularly token effort, given that Ram's latest full-sizers have thrown open the door for truly luxurious interior treatments in pickups. Given that the least-expensive Sierra HD AT4 and Denali models we've sampled have stickered for a well-optioned $66,610—our Denali test truck came to an eye-popping $78,455—GMC is well positioned to offer an even greater premium return for its discerning buyers.

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