GM has unveiled the GMC Sierra HD Concept ahead of its public debut in Detroit, and even though it hasn’t been confirmed yet, we suspect that this heavy-duty truck will make it into production.
Based on the 2011 Sierra HD, the GMC Sierra HD Concept uses a 6.6-litre Duramax turbodiesel V8 which produces 397 HP and 765 lb-ft of torque, and which is send power to the wheels via a 6-speed Allison 100 gearbox.
If compared to the 2011 Sierra HD, the GMC Sierra HD Concept is wider (73 inch/1,853 mm track) with a greater ground clearance of approximately 3 inches, and features custom front upper and lower control arms, specially developed off-road shocks, front and rear jounce shocks and an electronic front stabilizer bar disconnection.
The GMC Sierra HD Concept has also been fitted with 35-inch-tall BFGoodrich KM2 ‘mud terrain’ tires mounted on 20 inch aluminium wheels.  
In depth information regarding the GMC Sierra HD Concept is available in the official press release below.
GM press release :
DETROITThe GMC Sierra All Terrain HD concept
is an exploration of heavy-duty truck capability combined with greater
off-road versatility. It is highlighted by a bold exterior design,
enhanced all-terrain suspension and premium details expected of a GMC –
all designed to enable greater access to off-road destinations.
The new 2011 Sierra HD underpins the All Terrain HD concept, with a
modified, production-based 4WD chassis and the Duramax diesel/Allison
1000 six-speed powertrain. The enhanced suspension and unique body
dimensions, including increased ground clearance and wider track, as
well as greater approach/departure angles, deliver off-road capability
while maintaining HD levels of payload and trailering capacity.
“The Sierra All Terrain HD concept takes GMC’s outstanding new HD
platform to the next level, marries it to the legendary Duramax
diesel/Allison transmission, and provides a heavy-duty expression of
GMC’s premium All Terrain package,” said Lisa Hutchinson, GMC product
marketing director. “It delivers the exceptional capabilities of the
Sierra HD – hauling, towing and performance – and applies them to the
toughest driving environments. It’s the ultimate professional-grade tool
for construction crews, ranchers and adventurers whose activities
aren’t limited by where the pavement ends.”
The Sierra All Terrain HD features a unique five-foot, eight-inch
Crew Cab/short box body configuration that supports a
wheels-at-the-corners proportion. Shorter in overall length than
production Sierra HD models, the concept’s dimensions contribute to
greater approach/departure angles. Additional off-road elements include:

  • Wider, 73-inch (1,853 mm) track for greater stability
  • Approximately 3 inches (76 mm) greater ground clearance
  • Custom front upper and lower control arms
  • Specially constructed Fox off-road shocks with remote fluid reservoirs (integrated in the wheel house liners)
  • Front and rear jounce shocks
  • Electronic front stabilizer bar disconnection
  • 35-inch-tall BFGoodrich KM2 “mud terrain” tires mounted on 20-inch machined aluminum wheels
  • Full composite underbody protection.
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“The capability-enhancing attributes of the Sierra All Terrain HD
build on the already outstanding capabilities offered in the all-new
production Sierra HD trucks,” said Hutchinson. “Although it is strictly a
concept, it is a pretty realistic one.”
Purposeful design
From every angle, the All Terrain HD looks the part of a dedicated
off-road performer, with bold, accentuated features complemented by a
wide stance and tall profile. 
Streamlined bumpers are designed to enhance approach/departure angles
and incorporate frame-mounted recovery hooks – with integrated skid
plates. A large, airflow-optimized grille maximizes cooling in tough
conditions, while a forced-induction hood delivers more air to the
Duramax diesel engine.
“The design of the All Terrain HD is an expression of its
capabilities – strong, functional and absolutely professional grade,”
said Carl Zipfel, design manager. “We’re excited about the design
elements and exploring how they could apply to future GMCs.”
Large fender flares stretch away from the body to cover the
35-inch-tall BFGoodrich tires. At the rear, the cargo box features
lockable, lighted storage compartments, along with a number of tie-down
cleats, including a pair in the bed floor and two on each side of the
bed. A non-slip rubber pad is used on the tailgate, while two special
compartments inside the cargo box provide access to a 110-volt power
outlet and air compressor. A composite bed liner with a unique “circuit
board” pattern provides a protective cover for the bed.
Motorized, deployable assist steps for the cab and cargo bed make it
easier to enter and exit the All Terrain HD, while maximizing ground
clearance during off-road driving. It also incorporates a number of
industrial-grade lighting elements to provide greater visibility for
safer driving where stars may be the only other source of illumination.
The lighting details include:

  • High-intensity LED headlamp, taillamp and fog lamp
    lighting mounted in impact-resistant composite housings and
    featuring edge-lit illumination
  • High-intensity LED park and turn signal lamps
  • Smooth-appearance front marker lamps integrated behind the top edge of the windshield glass
  • Integrated center high-mounted stop lamp and rear marker lights mounted at the top edge of the rear window glass
  • LED floodlights for the cargo bed and assist steps.

The All Terrain HD is painted Iridium Metallic – a dark charcoal
color that reveals a reddish, anodized inflection when viewed from
certain angles. It complements other anodized and satin-metal finishes
on the exterior, which give the truck a more precise and functional
appearance.
The interior matches the exterior aesthetic, with premium
appointments and increased functionality, including enhanced lighting.
It conveys solidity and precision, blending accent details such as
stainless steel mesh and satin chrome with an amplified expression of
the All Terrain’s premium features. Two-tone dark gray leather seats,
matched with embossed carbon fiber-style leather on the seating
surfaces, have red contrast stitching.   Additional leather-wrapped
interior features include the steering wheel, instrument panel and
center console.
A new navigation radio is also featured on the Sierra All Terrain HD
concept and previews future radios to be offered in GMC vehicles. An
integrated 80-gig hard drive contains map data for the navigation
system. It eliminates the need for a map disc and provides greater
storage of music from portable devices. It also features DVR-style “time
lapse playback,” which allows up to 20 minutes of recording/playback
from the AM/FM/XM radio stations.
All Terrain chassis and suspension
Based on the chassis of the 2011 Sierra HD trucks, the All Terrain HD
features a fully boxed steel frame with exceptional strength and
torsional stiffness. It incorporates the production models’ new
independent front suspension and asymmetrical rear leaf-spring
suspensions, but with a wider 73-inch (1,853 mm) track (front and rear)
that enhances stability during off-road and highway driving. A
production Sierra 2500HD has front and rear tracks of 68.8 inches (1,748
mm) and 67.3 inches (1,709 mm), respectively.
An increased ride height provides greater ground clearance of 21.1
inches (536 mm) at the rockers and 11.8 inches (300 mm) at the skid
plates, while specialized Fox off-road shocks deliver exceptional
damping on tough terrain. They were created specially for the All
Terrain HD, with specific valving. The shocks feature visible, remote
fluid reservoirs – connected to the shock bodies via custom hoses –
mounted in the wheel house liners and are matched with front and rear
jounce shocks. They add additional compression damping and more
controlled rebound, which helps increase control, stability and
handling.
The All Terrain HD also features electronic disconnection of the
front stabilizer bar, which enables greater crawling capability on rocky
terrain. The driver simply pushes a button inside the cab to disconnect
it from the front suspension.
Custom upper and lower control arms for the front suspension are
designed to work with the Fox off-road shocks and jounce shocks, while
supporting the greater ground clearance and ride height. They contribute
to the wider track, as do the aggressive 20-inch machined aluminum
all-terrain wheels. The wheels are deep, with six split-spoke elements,
and are constructed in a reverse drop-flange method that ensures optimal
strength. They also feature a mix of satin chrome and anodized aluminum
finishes, which provide durability and a premium appearance that
complements the body’s Iridium Metallic color.
A full composite underbody protection system shields the All Terrain
HD’s transmission, drivetrain, fuel tank, exhaust system and axle
differentials. It starts with the front bumper, which rolls beneath the
truck and leads to a composite, integral belly pan that covers a front
suspension skid plate. Protective covers are also mounted beneath the
driveline components, while the front and rear differentials feature
direct-mounted skid plates. The rear bumper is integrated with the rear
skid plate.
Powertrain details
The Sierra All Terrain HD concept is propelled by the new, production
6.6L Duramax turbo-diesel V-8 and Allison 1000 six-speed automatic
transmission powertrain combination offered in the 2011 Sierra HD
trucks. The Duramax is rated at 397 horsepower (296 kW) and 765 lb.-ft.
of torque (1,037 Nm).
The powerful 6.6L Duramax is more fuel-efficient – with up to
11-percent greater fuel economy than previous versions – and reduces NOx
emissions by up to 63 percent. The powertrain’s efficiency is assisted
by the Allison 1000 transmission, which requires less engine power to
funnel torque to the axles. It also incorporates a “smart” exhaust brake
feature that helps save wear on the brakes on downhill grades, a
feature available on production Sierra HDs.
With the same proven powertrain as production Sierra HD trucks, the
All Terrain HD delivers comparable hauling and towing capabilities,
including:

  • Estimated payload capacity of 2,700 pounds (1,225 kg)
  • Estimated conventional towing capacity of 13,000 pounds (5,896 kg)
  • Estimated fifth-wheel towing capacity of 15,600 pounds (7,076 kg).
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The Sierra All Terrain HD also features four-wheel drive, with
automatic locking front and rear differentials, while the 14-inch,
four-wheel disc brake system from the production Sierra HD provides the
stopping power for this unique off-roader.
GMC SIERRA ALL TERRAIN HD CONCEPT SPECIFICATIONS

Vehicle:
GMC All Terrain HD concept
Body style / driveline: 5-passenger crew cab, 3/4-ton 4-wheel-drive heavy-duty pickup
Construction: body on frame
Engine: Duramax 6.6L turbo-diesel
Horsepower / kW @ rpm: 397 / 296 @ 3000
Torque (lb.-ft/ / Nm) @ rpm: 765 / 1037 @ 1600
Transmission: Allison 1000 six-speed automatic
Suspension (front): long- and short-arm independent with torsion bars
Suspension (rear): semi-elliptic two-stage multi-leaf spring
Steering type: integral power-assisted recirculating ball
Brakes (type): power-assisted, Hydroboost brake-apply system, four-wheel disc, four-wheel ABS
Brake rotor diameter x thickness
(in / mm):
front: 13.97 x 1.57 / 355 x 40
rear: 14.17 x 1.34 / 360 x 34
Wheel size and type: 20 x 9.5-inch aluminum
Tire type and size: BFGoodrich KM2 325/60R20
Wheelbase (in / mm): 148.6 / 3774
Overall length (in / mm): 230.9 / 5864
Overall width (in / mm): 83 / 2106
Overall height (in / mm): 81.8 / 2077
Track – front (in / mm): 73 / 1853
Track – rear (in / mm): 73 / 1853
Minimum ground clearance – (in / mm): 21.1 / 536 (at rocker panels);
11.8 / 300 (at skid plates)
Approach angle – front (deg.): 39
Approach angle – rear (deg.): 31
Maximum payload (lbs. / kg): 2700 / 1225 (est.)
Maximum towing capacity – ball hitch (lbs. – kg): 13,000 / 5896 (est.)
Maximum towing capacity – fifth-wheel hitch (lbs. – kg): 15,600 / 7076 (est.)
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Note: Information shown is current at time of publication.

Source: GM