SPRINGFIELD, Mo. -- The roly-poly U.S. 65 multi-slab slices through the Ozark Mountains of southern Missouri, and we're taking this asphalt artery to experience the big-time fire power and tire traction of the GMC Sierra 2500HD in an all-new 2015 iteration.
The "HD" initials signify Heavy Duty and describe a vast line of big and powerful work trucks developed by General Motors for the upward expansion of GMC's full-size line of Sierra pickups.
Dressed in new sheetmetal with revamped truck beds, revised cab designs and plush cabin content, the Sierra 2500HD and 3500HD trucks for model-year 2015 emerge on high-strength chassis to enhance the ride and handling with stronger suspensions and beefy brakes, options for two powerful engines and overall boosted capacity to perform truck-tough chores with greater payload capability and maximum available tow ratings -- 19600 pounds with a factory hitch and 23200 pounds for fifth-wheel towing.
In size-wise sequence GMC's big trucks begin with Sierra 2500HD, which differs from a light-duty Sierra because it rides on the stronger and longer HD chassis and pulls from one of two engines tapped for HD service.
The base Sierra 2500HD has the shortest wheelbase of the series -- 133.6 inches. It installs a regular-size 2-door cab and carries a pickup box that measures 97.8 inches in length.
There are choices for a gasoline-driven Vortec V8 engine or a Duramax massive-torque turbo-diesel V8. Both rear-motivated 2-wheel-drive and 4-wheel-drive traction modes apply.
GM's Vortec 6000 6.0-liter V8 serves as the gasoline-swigging plant. Output tallies to 360 hp at 5400 rpm, with torque skewed to 380 lb-ft at 4200 rpm. This engine ties to a heavy-duty Hydra-Matic 6L90 electronic automatic transmission with six forward gears.
The Duramax 6.6-liter V8 direct-injection turbo-diesel engine makes 397 hp at 3000 rpm and torque of 765 lb-ft at only 1600 rpm. The diesel links to a 6-speed electronic automatic Allison 1000 transmission which has a high-tech transmission control module to select gears and engage the torque converter lockup.
GMC Sierra HD trucks offer three cab styles -- Regular Cab (two doors and a bench or buckets), new Double Cab with two forward-hinged rear doors (bench or buckets in front of a back bench for three), and Crew Cab (4-door cab with front buckets or bench and big 3-seat back bench).
There are two different lengths for the rear truck bed -- 6'6" (78.9 inches) and 8' (97.8 inches), each with CornerStep bumpers and EZ Lift-and-Lower tailgate.
Sierra 2500HD and 3500HD trucks are constructed on fully boxed steel frame rails using high-strength steel. The suspension -- independent front with forged steel upper control arms and cast iron lower control arms plus torsion bars in place of springs to facilitate trim height adjustments, and rear asymmetrical multileaf springs -- produces a remarkably smooth ride quality.
The power-assisted brake system installs a disc at every wheel with Duralife brake rotors and linkage to a 4-wheel anti-lock brake system (ABS) and GM's StabiliTrak electronic skid controls plus standard Trailer Sway Control on all models including 3500HD dual-rear-wheel models.
An optional Z71 off-road suspension package adds Rancho monotube shocks on 2500HD, plus Hill Descent Control, underbody shields, brushed-metal door sill plates, a Z71-logo gauge cluster, special interior trim, unique front bumper and grille, and Z71-logo graphics on the cargo box.
GMC trims the 2015 Sierra 2500HD and 3500HD trucks as Sierra, SLE and SLT, plus a high-end version, Sierra HD Denali Crew Cab.
Unique gear for Sierra HD Denali includes projector-style headlamps with LED signature daytime running lights, a polished stainless steel exhaust outlet, body-color front and rear bumpers, chrome-lined assist steps, 20-inch polished aluminum wheels for 2500HD, heated and cooled leather front bucket seats, a heated steering wheel and power sliding rear window with defogger, MyLink connectivity with 8-inch touch screen, a Bose premium audio kit and front/rear park assist system.
The bottom line: GMC's 2015 Sierra HD trucks carry MSRP figures rising from $32,500 to $56,500.