Heavy-duty pickup trucks are intended to be workhorses rather than show ponies, but the 2021 GMC Sierra 2500HD and 3500HD do both. While the giant GMC’s top-of-the-line Denali model looks glamorous and is loaded with upscale features, its interior isn’t as luxurious as the fanciest Ford Super Duty or Ram HD. Still, the Sierra has a more refined diesel powertrain, with its Duramax engine developing 910 lb-ft of torque and capable of pulling up to 35,500 pounds. Unlike its Ford and Ram rivals, the GMC features an independent front suspension that provides responsive steering and impressive maneuverability. However, the largest versions are still far from agile compared with the regular Sierra 1500. While a host of ingenious towing assists and some innovative features help the 2021 Sierra HD stand out, it’s still not our favorite heavy-duty pickup.
What’s New for 2021?
For 2021, the heavy-duty Sierra receives a host of new or improved trailer-assistance features. There’s a new jack-knife alert that does what its name suggests and a trailer-length indicator that lets the driver know when nearby traffic might interfere with a connected trailer. The Multipro tailgate is now standard on the SLE trim level and there are three new paint colors. The Denali adds a newly standard high-definition 360-degree camera and a bed-view camera as well as a new Diamond Edition. The latter wears exclusive black 20-inch wheels and power-retractable running boards. Finally, the off-road-oriented AT4 is now available with mud-terrain tires and a high-clearance step accessory.
Engine, Transmission, and Performance
The Sierra HD comes standard with a gas-powered 6.6-liter V-8 that generates 401 horsepower and 464 lb-ft of torque. It pairs with a six-speed automatic transmission and rear- or all-wheel drive. The optional Duramax diesel 6.6-liter V-8 cranks out 445 ponies and 910 lb-ft of twist, and it works through an Allison 10-speed automatic. You can also specify either a single- or dual-rear-wheel axle (known as a “dually”). While we’ve only driven the diesel variant, we did sample the gas-burning powertrain on the internally identical Silverado 2500HD and appreciated its dutiful behavior. However, there’s no arguing that the Duramax/Allison combination is a highlight of the heavy-duty Sierra, and it’s a no-brainer for those who regularly tow substantial loads. The engine is surprisingly refined, and the transmission is very responsive. Together they helped the 2500HD Denali we tested achieve impressively quick acceleration times. The gigantic GMC also has an agreeable ride quality for its breed. Though its steering is vague at highway speeds the Sierra nonetheless feels more agile than its heavy-duty competitors. The trail-ready AT4 model is a more-rugged take on the commonly chrome-riddled rig, with an off-road suspension and a locking rear differential to go with its otherwise blacked-out aesthetic.
Towing and Payload Capacity
The strongest Sierra 3500HD can carry 7442 pounds in its cargo bed and pull as much as 35,500 pounds. However, those massive capacities require the diesel engine, regular cab, rear-wheel drive, and dual-rear-wheel axle. The Sierra 2500HD is only offered with a single-rear-wheel axle, and its burliest version is good for 3979 pounds of payload and 18,500 pounds of towing.
Fuel Economy and Real-World MPG
Since the EPA doesn’t test vehicles as heavy as the Sierra HD and its gigantic competitors, we can only judge their real-world mpg on our highway fuel-economy route. We’ve only tested a Sierra 2500HD Denali with the diesel engine, which earned 17 mpg on the highway. For comparison, the diesel-powered Silverado 2500 LTZ crew cab and Ram 2500 Tradesmen regular cab diesels we tested both managed 19 mpg. Those who stick with the GMC’s standard gas-fed V-8 should expect to see even worse fuel economy than the diesel option.
Interior, Comfort, and Cargo
Inside, the Sierra 2500HD and 3500HD have a huge dashboard that hosts user-friendly switchgear. Most models have a split-folding front bench that can be replaced with a pair of bucket seats and a full center console. The Sierra’s interior can also be optioned with a class-exclusive head-up display, a heated steering wheel, heated-and-cooled front seats, and wireless charging. Unfortunately, the cabin’s panel fits and material quality are mediocre at best. While there’s an abundance of soft touch surfaces throughout, and most models benefit from durable plastics, the top-of-the-line Denali falls woefully short of its luxury billing. The fanciest alternatives from Ram overshadow the GMC largely thanks to their significantly more upscale look and feel. Still, the Sierra HD will please pickup-truck buyers who prioritize practicality and versatility. Along with extensive passenger space in the crew cab, the GMC is littered with handy interior cubby storage. Its standard and long cargo boxes have built-in steps on both sides of the bed and the rear bumper has them at each corner. The box can be further customized with LED lighting, movable tie-downs, and a 120-volt outlet. The heavy-duty Sierra’s biggest showpiece is its innovative multi-function tailgate, which has six different positions that include a work surface and bench. It’s a tailgater’s dream come true.
Infotainment and Connectivity
Every Sierra HD has a standard touchscreen infotainment system with Apple CarPlay and Android Auto. While base models have fewer options, they can still be outfitted with a 120-volt outlet and a mobile hotspot. Most trim levels have an 8.0-inch screen and additional charging ports. All systems include a set of rotary knobs and physical buttons that are easily accessible. The larger touchscreen can be upgraded with built-in navigation, a Bose audio system, SiriusXM satellite radio, and a rear-seat entertainment system. GMC also offers a smartphone app that can be used to remotely control trailer functions. Likewise, the truck’s infotainment system allows users to store different trailer profiles to monitor maintenance.
Safety and Driver-Assistance Features
The 2021 Sierra HD hasn’t been crash tested by the National Highway Traffic Safety Administration (NHTSA) or the Insurance Institute for Highway Safety (IIHS). While the truck can be equipped with various driver-assistance technology, none of it comes standard, and heavy-duty rivals offer even more assists. However, the GMC does have a comprehensive camera system that features up to 15 different viewing modes, including a “transparent” mode that allows the user to virtually see through an attached trailer as if it wasn’t there. Key safety features include:
- Available forward-collision warning and automated emergency braking
- Available blind-spot monitoring and rear cross-traffic alert
- Available lane-departure warning