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At the start of the day the trucks are spotless, their 18-inch wheels, dark-finish exterior trim and red tow hooks on prominent display. A couple of hours later they're filthy, splattered with dirt and dripping muddy water. That is, after all, exactly how the 2019 GMC Sierra AT4 is meant to be used. It's a package designed for Sierra buyers who plan to take their truck off-roading, and a romp around the trails at Eaton's proving grounds in Marshall, Michigan, reveals that the AT4 is more than up to the task.
Eaton opened its gates to a group of journalists driving GMC Sierras because the supplier manufactures the AT4's locking rear differential. The company's 636-acre proving ground has plenty of space for romping around in the mud and dirt, as well as various paved facilities for testing everything from everyday vehicles to Class 8 trucks. But as you'll be able to tell from our video, the "proving grounds" felt more like "playground" on my visit.
AT4 is a new trim level for the 2019 GMC Sierra that amps up its off-pavement abilities. The truck receives a 2-inch suspension lift complemented by Rancho shock absorbers, the aforementioned Eaton G80 locking diff and either 18- or 20-inch wheels, the former of which can be outfitted with Goodyear Wrangler Duratrac mud-terrain tires. In other words, all the types of upgrades off-roaders might want in order to take their truck out into the woods.
The AT4 comes standard with a 5.3-liter V8, which is rated for 355 horsepower and 385 pound-feet of torque and is mated to an eight-speed automatic transmission. Buyers can upgrade to a 6.2-liter V8 with 420 hp, 460 lb-ft and a 10-speed automatic. And AT4 buyers can also opt for a performance air intake and exhaust kit that'll unlock even more power: an extra 12 hp and 6 lb-ft on the 5.3 V8 and 15 hp/9 lb-ft for the 6.2 V8. But you'll really want that option for the boisterous, rumbly sound it produces under heavy throttle.
Other features include hill descent control, standard four-wheel drive, dark-finish exterior trim, a body-color grille and bumper, vertical-oriented red recovery hooks, "Kalahari" seat inserts and rubber floor mats. The AT4 is also one of only two trim levels (the other being the glitzy Denali) that will be offered with the Sierra's optional CarbonPro carbon-fiber bed. GMC has said it will roll out the AT4 treatment to other models in its lineup, though we'll have to wait to see what the next model will be. (The Canyon midsize pickup truck seems a likely candidate.)
I started out by tackling a handful of obstacles that Eaton uses specifically to test vehicle capability. A big ditch, for instance, is intended to flex a truck's frame and also requires generous approach and departure angles so as not to scrape body parts. I easily drove the AT4 in and out of the trench without issue, the truck's 2-inch lift helping and the immense gearing advantage of the four-wheel-drive system's low range making ascending the other side a breeze.
Blasting through a deep mud pit was easy too. The Eaton differential came in handy here. It self-locks the rear axle as soon as one wheel spins 100 rpm faster than the other -- which, given how much the truck slipped and slid and kicked up mud, must have been pretty much instantly. A "low traction" warning flashed on the head-up display but without much fuss, the truck powered out the other side. (Another journalist did get stuck in the mud pit and had to be towed out by a Caterpillar earth-mover, but we can probably ascribe that to operator error.)
Next, clomping up a hill formed of big rocks was, if not comfortable, not particularly harsh as the Rancho shock absorbers smoothed out big up-and-down motions. Here I was particularly grateful for the AT4's skid plates and the fact that, unlike the truck I drove on the road, these ones don't have running boards that could snag on obstacles.
The so-called Woods Route is more like what you might find out in real-world off-roading, with a narrow trail that wends between trees and up and down small hills. Slippery leaves and patches of mud from recent snow melt posed no real hassle to the AT4. Nor did clambering up a technical hill strewn with big rocks against which I hoped to avoid smashing the truck's fog lights. But the sure-footed Sierra frankly turned the whole thing into a non-vent, so plentiful was its grip and so ample its ground clearance.
An optional color HUD display has a special off-roading mode showing steering, roll and incline angles. The 360-degree camera system has a dozen or show vantage points, including ones pointing directly at the truck's corners so you can see exactly where you're positioning the tires. That's nice to have on a truck that's this long and wide.
On the nearly 2-hour highway drive back home from Eaton's course, the Sierra AT4 showed little evidence it had been modified for off-pavement excursions. The knobbly tires did thrum on the road, even at neighborhood speeds, but the suspension and steering were just as solid and stable-feeling as a regular Sierra. What was noticeable, however, was that other motorists could tell this truck was special: More than once I spotted other Sierra or Chevy Silverado owners craning their necks and doing a double-take at the AT4.
The only major demerit is EPA ratings of 15 miles per gallon city and 19 mpg highway, even with the V8 offering cylinder deactivation and stop-start -- and with premium fuel recommended, no less. I found only the gentlest cruising right at the speed limit would get me close to the highway number. Still, those numbers are par for the course with big trucks like this.
The 2019 GMC Sierra AT4 starts at $53,200 before destination or options, and the latter can quickly add up. My truck had an as-tested price of $65,475. Options include upgrading from the 5.3- to the 6.2-liter V8 ($2,495), a $995 sunroof and a $1,595 Performance Exhaust kit. My truck also featured the $3,280 AT4 Premium package, which adds an 8-inch infotainment system with navigation, wireless phone charging, a power rear window, parking sensors, blind-spot monitoring and side steps. The $745 Driver Alert Package II adds precollision braking with pedestrian detection, lane-keep assist and automatic high beams. Tack on a couple more features, like the surround-vision camera, and it's easy to see how the truck's sticker price can escalate.
The GMC Sierra AT4 is not a go-anywhere rock-crawler, like the Chevy Colorado ZR2, or a high-speed desert blaster, like the Ford F-150 Raptor. However, as playing around on Eaton's course proved, it's plenty capable for the sort of mild-to-moderate off-roading that a real owner might want to do at the weekend. Better still, the AT4 treatment doesn't significantly compromise the truck the rest of the time, making it a solid choice for anyone who wants to take the GMC Sierra beyond the pavement.