2020 GMC Sierra 2500 AT4 Diesel: Real-World Fuel Economy

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GMC added the AT4 off-road trim to the redesigned 2020 Sierra heavy-duty lineup. We recently took possession of a loaded 2020 Sierra 2500 AT4 crew cab and wondered just how its fuel economy would stack up on our southeast Michigan test loop. The answer surprised us.

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My test truck came with the stout 6.6-liter Duramax diesel, making a familiar 445 horsepower and 910 pounds-feet of torque, but it was mated to a new 10-speed Allison heavy-duty automatic transmission. Part-time four-wheel drive is part of the AT4 package; the test was conducted in two-wheel drive. I set out from Ann Arbor, Mich., on a perfect day — 60 degrees, no wind, no rain, just a perfect autumn morning. My route took me through urban stop-and-go traffic, suburban street grids and a good deal of interstate highway. Speeds were kept to within 5 mph of posted limits, and acceleration and braking were performed as smoothly and consistently as possible, as aggressive driving is the enemy of fuel economy. Tire pressures were set to the manufacturer’s recommended levels while cold.

The Results

Over a total of 208.1 miles, the Sierra consumed 10.453 gallons of diesel fuel. This averaged to a calculated 19.91 mpg combined. The truck’s trip computer was a bit more conservative, telling me it had achieved 19.6 mpg. On a similar route, the 2019 Nissan Titan XD Pro-4X returned almost 19 mpg, and a Ford F-250 XL work truck with gas 6.2-liter V-8 did about 16 mpg.

The AT4 delivers this impressive fuel economy in a package that doesn’t feel like an off-road truck. You can get an AT4 as a 2500 or 3500 single-rear-wheel truck, which includes a unique off-road suspension with Rancho shocks, more skid plates, an Eaton locking rear differential, a specific Off-Road mode for the traction control system, plus electronic hill descent control and hill start assist, and either 18- or 20-inch wheels with off-road tires. It’s not as extensive a package as a Ram 2500 Power Wagon, but it will be an interesting competitor to the upcoming 2020 Ford Super Duty F-250 Duty Tremor.

It doesn’t really feel like a heavy-duty either. The latest GMC Sierra 2500 is sharp with a surprisingly smooth ride, stable steering and impressive body control. It’s a massive truck, yes, but its driving dynamics belie its size — it feels more maneuverable and responsive than its massive nearly 8,000-pound curb weight would suggest. Brakes bite and haul the truck down quickly. There’s an impressive absence of wind and road noise from the cabin. And even the cabin’s unique AT4 leather trim and stitching looks better than most of the other GM truck cockpits, and while it’s not quite to the level of the premium materials found in a Ram Power Wagon, it’s still on the nicer end of the GM truck spectrum.

A shade less than 20 mpg in an HD truck is an excellent result, made even more astonishing knowing that it was observed in the off-road variant. The aggressive chunky-tread off-road tires, the extra weight from underbody protection, four-wheel drive and beefier suspension components are all enemies of good fuel economy.’s Editorial department is your source for automotive news and reviews. In line with’s long-standing ethics policy, editors and reviewers don’t accept gifts or free trips from automakers. The Editorial department is independent of’s advertising, sales and sponsored content departments.

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Detroit Bureau Chief Aaron Bragman has had over 25 years of experience in the auto industry as a journalist, analyst, purchasing agent and program manager. Bragman grew up around his father’s classic Triumph sports cars (which were all sold and gone when he turned 16, much to his frustration) and comes from a Detroit family where cars put food on tables as much as smiles on faces. Today, he’s a member of the Automotive Press Association and the Midwest Automotive Media Association. His pronouns are he/him, but his adjectives are fat/sassy. Email Aaron Bragman

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