The GMC Sierra 2500HD III, specifically the Extended Cab Long Box configuration powered by the 6.6-liter Duramax turbodiesel V8 and paired with an Allison automatic transmission, represents a significant iteration in GMC’s heavy-duty truck lineage. Produced from 2011 to 2014 as part of the GMT900 platform’s facelift, this variant was aimed squarely at the American market’s demand for capable, powerful work trucks. It slotted into the lineup as a premium offering within the 2500HD range, positioned above the gasoline-powered engines but below the more specialized configurations like dual-rear-wheel models. This truck was designed for owners needing substantial towing and hauling capacity without sacrificing a degree of everyday usability.
Technical Specifications
| Brand | GMC |
| Model | Sierra 2500HD |
| Generation | Sierra 2500HD III (GMT900, facelift 2011) Extended Cab Long Box |
| Type (Engine) | 6.6 Duramax TD V8 (397 Hp) Automatic |
| Start of production | 2011 |
| End of production | 2014 |
| Powertrain Architecture | Internal Combustion engine |
| Body type | Pick-up |
| Seats | 5 |
| Doors | 2 |
| Fuel Type | Diesel |
| Weight-to-power ratio | 8 kg/Hp, 125.3 Hp/tonne |
| Weight-to-torque ratio | 3.1 kg/Nm, 327.3 Nm/tonne |
| Power | 397 Hp @ 3000 rpm |
| Power per litre | 60.2 Hp/l |
| Torque | 1037 Nm @ 1600 rpm (764.85 lb.-ft. @ 1600 rpm) |
| Maximum engine speed | 3000 rpm |
| Engine layout | Front, Longitudinal |
| Engine Model/Code | Duramax LML |
| Engine displacement | 6599 cm3 (402.7 cu. in.) |
| Number of cylinders | 8 |
| Engine configuration | V-engine |
| Cylinder Bore | 103 mm (4.06 in.) |
| Piston Stroke | 99 mm (3.9 in.) |
| Compression ratio | 16:1 |
| Number of valves per cylinder | 4 |
| Fuel injection system | Diesel Commonrail |
| Engine aspiration | Turbocharger, Intercooler |
| Valvetrain | OHV |
| Engine oil capacity | 9.5 l (10.04 US qt | 8.36 UK qt) |
| Coolant | 24 l (25.36 US qt | 21.12 UK qt) |
| Kerb Weight | 3168 kg (6984.24 lbs.) |
| Max. weight | 4536 kg (10000.17 lbs.) |
| Max load | 1368 kg (3015.92 lbs.) |
| Trunk (boot) space – maximum | 2138 l (75.5 cu. ft.) |
| Fuel tank capacity | 136 l (35.93 US gal | 29.92 UK gal) |
| Permitted trailer load with brakes (12%) | 5897 kg (13000.66 lbs.) |
| Length | 6337 mm (249.49 in.) |
| Width including mirrors | 2032 mm (80 in.) |
| Height | 1971 mm (77.6 in.) |
| Wheelbase | 4018 mm (158.19 in.) |
| Front track | 1748 mm (68.82 in.) |
| Rear (Back) track | 1709 mm (67.28 in.) |
| Ride height (ground clearance) | 216 mm (8.5 in.) |
| Minimum turning circle (turning diameter) | 15.8 m (51.84 ft.) |
| Drivetrain Architecture | Rear wheel drive |
| Number of gears and type of gearbox | 6 gears, automatic transmission Allison |
| Front suspension | Independent torsion bar, double wishbone |
| Rear suspension | Rigid axle suspension, Leaf spring |
| Front brakes | Ventilated discs, 350×40 mm |
| Rear brakes | Ventilated discs, 360×34 mm |
| Assisting systems | ABS (Anti-lock braking system) |
| Steering type | Cone worm with recirculation balls |
| Tires size | 245/75 R17; 265/70 R17; 265/70 R18; 265/60 R20 |
| Wheel rims size | 7.5J x 17; 8J x 18; 8.5J x 20 |
Powertrain & Engine Architecture
The heart of this Sierra 2500HD is the Duramax LML 6.6-liter V8 turbodiesel. This engine represents a significant evolution of the Duramax design, incorporating a redesigned combustion chamber, improved fuel injection strategies, and a more robust cooling system. The LML utilizes a common-rail direct injection system, delivering fuel at extremely high pressures for optimized combustion efficiency and reduced emissions. The engine also features a variable geometry turbocharger (VGT) and an intercooler to maximize power and torque across the engine’s operating range. The overhead valve (OHV) valvetrain, while seemingly traditional, is known for its durability and simplicity. The Allison 6-speed automatic transmission is a heavy-duty unit specifically designed to handle the substantial torque output of the Duramax engine. It features a lock-up torque converter for improved fuel economy at highway speeds and is known for its reliability and smooth shifting characteristics.
Driving Characteristics
The 397 horsepower and 765 lb-ft of torque produced by this combination translate into impressive real-world performance. While not as quick off the line as some gasoline-powered trucks, the Duramax/Allison pairing delivers a broad, flat torque curve, making it exceptionally capable for towing and hauling heavy loads. The automatic transmission is well-tuned for this application, providing smooth and predictable shifts. Compared to the 6.0L gasoline V8 option, the Duramax offered significantly more torque and better fuel economy, especially when towing. While a manual transmission was available on some 2500HD models, the Allison automatic was favored by many drivers for its ease of use and reduced driver fatigue, particularly in demanding work applications. The gear ratios in the Allison transmission are optimized for both pulling power and highway cruising.
Equipment & Trim Levels
The 2011-2014 Sierra 2500HD Extended Cab Long Box with the Duramax engine typically came standard with features like air conditioning, power windows and locks, a basic audio system, and vinyl upholstery. Higher trim levels, such as the SLT and Denali, added features like leather upholstery, heated seats, a premium audio system, navigation, and a rear vision camera. The Extended Cab configuration provided additional interior storage space behind the front seats, making it more versatile for carrying passengers and cargo. Optional extras included a trailer brake controller, a fifth-wheel hitch, and various appearance packages.
Chassis & Braking
The Sierra 2500HD features a robust chassis designed to withstand the stresses of heavy-duty use. The front suspension utilizes an independent torsion bar setup with double wishbones, providing a comfortable ride and good handling. The rear suspension employs a traditional leaf spring design, optimized for load-carrying capacity. Braking duties are handled by ventilated disc brakes on all four wheels, providing ample stopping power even when towing. An Anti-lock Braking System (ABS) is standard. The heavy-duty chassis and braking system are crucial for managing the truck’s substantial weight and towing capacity.
Market Reception & Comparison
The Duramax-equipped Sierra 2500HD was generally well-received by critics and consumers alike. It was praised for its powerful engine, smooth-shifting transmission, and impressive towing capacity. Fuel economy, while not exceptional, was considered good for a truck of its size and capability. Compared to the Ford Super Duty and Ram 2500, the Sierra 2500HD offered a competitive combination of power, refinement, and reliability. Some reviewers noted that the interior, while functional, wasn’t as luxurious as some of its competitors. However, the Sierra’s reputation for durability and long-term reliability helped it maintain a strong position in the heavy-duty truck market.
Legacy
The Duramax LML engine and Allison transmission pairing have proven to be exceptionally durable and reliable powertrains. Well-maintained examples of the 2011-2014 Sierra 2500HD continue to command strong resale values in the used car market. These trucks are highly sought after by owners who need a capable and dependable work truck. While some maintenance items, such as the diesel particulate filter, can be costly to repair, the overall reliability of the powertrain is generally excellent. The GMT900 platform Sierra 2500HD remains a popular choice for those seeking a robust and capable heavy-duty truck that can handle demanding tasks for years to come.
