General Motors has been sitting on the sidelines when it comes to offering a serious off-road package in a full-size pickup. Ford is on the second generation of its amazingly capable F-150 Raptor—a truck that’s been thrilling 4WD enthusiasts for almost a decade. Similarly, Ram is on the second generation of its excellent 1500 Rebel off-road model and on the third-generation of its Power Wagon. Sure, Chevrolet has the talented midsize ZR2 pickup. But full-size fans have been left out. Until now.
GM’s Chevrolet Silverado and GMC Sierra light-duty trucks were redesigned for 2019. And these machines are packed with a host of new innovative features. However, four-wheel drive enthusiasts are apt to look past all of that newfangled tech and focus right on the hardware that gets them down the trail. And for the first time, the full-size Silverado and Sierra receive an off-road package that goes beyond anything offered before.
The Chevrolet Silverado Trail Boss and GMC Sierra AT4 add a two-inch lift and 32-inch Goodyear Duratrac tires to the Z71 and X31 (for GMC) grouping of parts. That package includes Rancho shocks, a two-speed transfer case, locking rear diff, skid plates, hill descent, high-capacity air cleaner, and an integrated dual exhaust. Unlike the Trail Boss which starts with a V6, GMC’s AT4 comes standard with a 5.3-liter V8. And, it can be optioned with the new 3.0-liter turbo-diesel straight six-cylinder or the stout 420 horsepower 6.2-liter V8 and 10-speed automatic.
We trained our sights on the top engine. And GMC delivered a fairly loaded $65,850 AT4 equipped with the 6.2-liter V8. So, we ran the big 4X4 through our typical evaluation process which includes plenty of time pounding the pavement as well as a half-day testing the limits of its tires, gearing and suspension on dirt trails. Does the AT4 offer the off-road chops it promises? Let’s find out.
The Hardware
Weight savings was a key parameter when designing the new Silverado and Sierra. And the company says the new, fully-boxed frame is 10 percent stiffer in torsional rigidity and hits the scales 88 pounds lighter too. Eighty percent of that frame is comprised of high-strength steel. That’s impressive considering the frame is longer than before. Our Crew Cab short bed (5-foot, 8-inch) GMC rides on a 147.5-inch wheelbase which is exactly four inches longer than last year’s truck. Other dimensions have increased as well. Width is up by just over an inch and our GMC AT4 towers over the previous-generation 4X4 with a 5-inch increase in height.
Up front, the AT4 uses a redesigned double-wishbone coil-sprung IFS with forged upper control arms. The AT4s extra two inches of suspension lift at the front end comes from longer Rancho dampers and new jounce bumpers. There are also specific half shafts too as well as a new electronic power steering calibration. At the rear end, the AT4 uses a lift block under the leaf packs as well as longer Rancho shocks. The 2-inch lift isn’t limited to AT4 or Trail Boss trucks. A normal Sierra or Silverado owner can order this suspension right at the dealership parts counter and have it installed in the service bay. GMC says the $1,295 4WD kit requires about 5 hours to install and the $995 2WD version takes just over 4 hours.
The axles and differentials are largely the same as the ones used in previous model trucks. Up front is an 8.6-inch open differential with a 9.5-inch solid axle in the rear. The AT4 doesn’t offer unique traction-adding differentials. Instead our truck used the same Eaton MLocker rear diff that GM trucks have used for decades. This unit uses a flywheel mechanism and a clutch system to engage when traction is compromised. Eaton says it can activate when a wheel speed difference of 100 rpm is detected.
Incredibly, GM offers a range of six powertrains for the new Silverado and Sierra. But when it comes to the AT4, GMC only offers the top engines. Our truck’s 6.2-liter V8 would normally produce 420 horsepower at 5,600 rpm and 460 lb-ft of torque at 4,100 rpm. But this one had a new Off-Road Performance Package, which includes a new exhaust and freer-flowing cold air intake to bump horsepower to 435 and torque to 469 lb-ft. Regardless of engine tune, the 6.2-liter comes paired to a ten-speed automatic—the same one Ford uses in its fullsize trucks. Here, it offers a deep 4.70:1 first gear and overdrive ratios in gears eight through ten. This drivetrain combination is only available with a 3.23:1 axle ratio.
Of course, since this is a specialized off-road model, our truck came with a traditional two-speed transfer case activated with dash buttons for 2WD, Auto 4WD, 4WD High and a 2.72:1 low-range ratio. But beware when buying a “4WD” truck from GM these days. The standard “4X4” Silverado or Sierra comes with a single-speed all-wheel drive system. Yes, you read that correctly. In order to get a proper two-speed transfer case, you must at least order the Z71 (Chevy) or X31 (GMC) package. And to be blunt, that sucks because it means the end for a bare-bones GM work truck with real 4WD. It also promises to be tedious for second-hand buyers of these trucks, because many “4X4s” won’t really be true 4X4s.
At our local scales the AT4 weighed 5,460 pounds. The last GM truck we tested with a similar 6.2-liter powertrain and cab configuration (a 2015 Silverado Midnight Edition) hit the scales at 5,660 lbs. So, this new truck checks in exactly 200 pounds lighter, which is significant. Our AT4 was rated to carry a payload of 1,432 pounds and tow an 8,000-pound trailer.
On the Street
The styling of the new Chevrolet Silverado has been polarizing. It’s a huge departure from the old truck and not a look we’re particularly fond of. Ugly may be a strong word but it’s certainly not handsome. To these eyes, the GMC Sierra (particularly in this AT4 trim) is a far better-looking truck and the best evidence yet for selecting a GMC over a Chevy.
One of the coolest features of the AT4 is actually the tailgate. Surprised? Us too. The Multipro tailgate was a skunkworks project inside GM for many years before debuting on these 2019 trucks. The gate has six positions including a middle portion that can ease loading all sorts of stuff. There’s even a built-in bed-extender that’s far less cumbersome than those flip back cages from decades ago. And the built-in step to help you get into the truck requires much less work to unfold than the one Ford offers. Overall, the tailgate is a super smart design that really boosts the capability of the bed.
Slide into the cab and anyone familiar with the last-generation GMC truck will be right at home. The design and furnishings inside look quite a bit like the old truck. While other truck makers are pushing huge screens and rich materials, this nearly $70,000 truck seems a little dated and low rent by comparison. The infotainment screen is on the small side and clunky to use. But we definitely dig all the camera views available on this GMC. A driver information center is found between the gauges, including an “off-road” inclinometer. That information is conveniently repeated in the truck’s head’s up display. But how exactly you get to all those functions isn’t exactly intuitive. Climb into the Crew Cab’s rear seat and it’s a cavernous space. Thanks to the wheelbase stretch, legroom is up by 2.5-inches making it an incredibly comfy place to ride. Many of the GMC’s interior criticisms are forgotten as soon you feel the 6.2-liter V8 under your right foot. This 435-horsepower engine makes for one quick truck. Our friends at Car and Driver tested an AT4 and found it to hit 60 mph in just 5.8 seconds. But that was without the bump in power afforded by the Off-road Performance Pack in our truck. This has to be one of, if not the best drivetrains on any truck. It simply hauls and it sounds mean too. The Terrain Mode dial has a Sport setting, which we found to sharpen up the throttle and shifting.
It’s rare that a lifted truck results in a better ride. But this GMC was a complete surprise. The truck was magic carpet-smooth on the road. Around town or on the highway on portions of the freeway that usually make a pickup truck “buck,” the AT4’s ride was supple and comfortable. This could be one of the best-riding 4X4s on pavement. And the GMC’s quiet cabin meant that you barely heard those knobby Duratracs—tires that we’ve found to be somewhat loud in other rigs. In all, the pavement driving experience in the AT4 was nothing short of excellent.
The AT4 was a relaxed cruiser too. On flat ground, the truck could shuffle along at 70 mph with just 1,500 rpm showing on the tach. A small grade would cause a downshift and maybe add another 150 rpm on the tach. The steepest grades only required one more downshift with 2,000 rpm showing on the tach. The torque of the engine is fantastic and it feels as though it could tow a trailer up this grade and maintain that same RPM and speed.
The GMC’s window sticker says it should return 15 mpg city and 19 on the highway. After our 300-mile on and off-road trek, the AT4 delivered 16.6 mpg. That’s almost 2 mpg less than the last-generation Silverado we tested with this same engine. Part of that is due to the raised suspension that creates a taller, less aerodynamic profile and part probably comes from our own enjoyment of the powertrain. Still it’s a more efficient machine than the last Raptor we tested which returned 15 mpg on a similar route and day of testing.
On the Trail
Speaking of the Raptor, the AT4 isn’t supposed to be an all-conquering competitor to that Ford. It’s a much milder take on an off-road package. So, our expectations for this GMC were certainly tempered. But in just about every instance, we were surprised by this truck’s performance.
The AT4 models still have a front airdam, but in this case its pulled back much further towards the tires and sits just over 11 inches above the ground. Better still, there’s almost 11 inches under the beefy skidplate protecting the front differential and almost 10-inches beneath the rear axle. Good number to be sure. Still, we expected to experience some rubbing on the harder trails around the park, but that wasn’t the case.
The AT4 impressed us early on by tackling some mild axle twisting climbs in 2WD. Just stay in the throttle long enough for the rear locker to engage, and combined with the chunky mud tire, traction was superb. We also noticed that compared to many pickups we’ve recently tested, the AT4 has an above average amount of rear axle articulation. The GMC does have a specific “off-road” setting in the Terrain Mode dial that alters throttle, ABS, traction control and stability control. But this mode is only available in 2WD and 4WD high range. It seems be a mode similar to a “mud” or “sand” mode in other 4X4s, allowing more tire slip to help maintain forward progress. Shift into low range and the GMC shifts out of that mode and into one that calibrates those electronics for crawling.
The AT4 was a surprisingly good low-speed four wheeling. On the toughest of the near 30-degree, rutted hill climbs we normally use for testing, the AT4 was not able to elegantly inch its way up without wheel slip, like some of the best 4X4s can. Instead, the GMC requires the driver to stay in the throttle a bit longer to make sure the rear diff locks. Then, with some mild tire speed, it clawed its way to the top. It was a good, if somewhat ungraceful, performance. And, it was one that most trucks we test here cannot accomplish because they simply don’t have the traction.
On our most challenging rutted mogul field the AT4 was able to crest the top without so much as kissing that airdam. We got very lucky because in other parts of the park we found a limiting factor of the AT4’s capability comes down to that low-hanging aero chin spoiler. And, like the hill climb, the GMC required a bit of throttle to maintain forward progress. Attempting to crawl these obstacles elegantly just isn’t possible. It takes a spinning tire for both the rear diff to send torque across the axle and for the traction control to engage. This AT4 may not be an elegant, but it is capable for a factory 4X4.
The AT4’s sweet-looking 18-inch “Machined Aluminum” wheels are wrapped with aggressive 275/65R18 Goodyear Duratracs. The 32-inch tires did an excellent job on every surface we tested them on. We’d definitely skip the 20-inch wheel and mild all-terrain tire option available on the AT4.
The Bottom Line
The AT4 is a new benchmark for GM’s full-size trucks when it comes to off-road capability. It’s a solid four-wheeler that’s equally at home on the street as a comfortable daily driver and workhorse. And that engine is simply one of the best around.
However, on an off-road focused truck that costs over $65,000, a more sophisticated electronic locking rear differential should be part of this package. Eaton’s automatic unit just isn’t as smooth or confidence inspiring as a selectable locker. And, as an off-road focused full-size light-duty pickup truck, this one isn’t inexpensive. A well-equipped Ford Raptor can be had for the same price as this truck.
Still, we like that GM is finally getting serious about four-wheeling. First, with the Colorado ZR2 and now with the Chevy Trail Boss and GMC AT4. But this is just the beginning. What’s next for GMC? The AT4 brand and philosophy is going to be applied to other models within GMC going forward. An AT4 version of the new Sierra HD already exists. So, we can’t wait to see what they unleash next.
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