The GMC Sierra 3500HD III, specifically the Extended Cab Long Box configuration with the 6.6-liter Duramax Turbo Diesel V8 engine paired with an Allison automatic transmission, represents a significant iteration in GMC’s heavy-duty truck lineage. Produced from 2011 to 2013 as part of the GMT900 platform’s facelift, this variant was aimed squarely at the demanding needs of professional tradesmen, ranchers, and those requiring substantial towing and hauling capabilities. It occupied a premium position within the Sierra 3500HD lineup, offering a balance of power, refinement, and work-ready functionality.
Technical Specifications
| Brand | GMC |
| Model | Sierra 3500HD |
| Generation | Sierra 3500HD III (GMT900, facelift 2011) Extended Cab Long Box |
| Type (Engine) | 6.6 Duramax TD V8 (397 Hp) SRW Automatic |
| Start of production | 2011 |
| End of production | 2013 |
| Powertrain Architecture | Internal Combustion engine |
| Body type | Pick-up |
| Seats | 5 |
| Doors | 2 |
| Fuel Type | Diesel |
| Weight-to-power ratio | 7.7 kg/Hp, 129.9 Hp/tonne |
| Weight-to-torque ratio | 2.9 kg/Nm, 339.2 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 | 3057 kg (6739.53 lbs.) |
| Max. weight | 5080 kg (11199.48 lbs.) |
| Max load | 2023 kg (4459.95 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 | 2436 mm (95.91 in.) |
| Height | 1966 mm (77.4 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) | 221 mm (8.7 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, 355×40 mm |
| Rear brakes | Ventilated discs, 373×34 mm |
INTRODUCTION
The 2011-2013 GMC Sierra 3500HD with the Duramax 6.6L diesel and Allison automatic transmission represented a significant refresh of the GMT900 platform. This generation of Sierra HD trucks was designed to compete directly with Ford’s Super Duty and Ram’s heavy-duty offerings. The facelift brought updated styling, improved interior materials, and enhanced technology. The Extended Cab Long Box configuration provided a useful blend of passenger space and cargo capacity, making it a popular choice for both work and personal use. This particular variant, with its potent diesel engine and robust transmission, was positioned as a premium offering for those needing serious towing and hauling capability.
Powertrain & Engine Architecture
At the heart of this Sierra 3500HD lies the Duramax LML 6.6-liter turbocharged diesel V8. This engine is a testament to diesel engineering, featuring a cast iron block, aluminum cylinder heads, and a robust crankshaft. The LML iteration of the Duramax introduced a revised combustion chamber design and a cooled exhaust gas recirculation (EGR) system to further reduce emissions while maintaining power output. The engine utilizes a high-pressure common rail fuel injection system, delivering fuel directly into the cylinders at extremely high pressures for optimal combustion. The valvetrain employs an overhead valve (OHV) configuration, known for its durability and simplicity.
The Allison 1000 six-speed automatic transmission is a critical component of this powertrain. Known for its strength and reliability, the Allison transmission is specifically designed to handle the high torque output of the Duramax diesel. It features a torque converter with a lock-up clutch to improve fuel efficiency at cruising speeds. The transmission’s gear ratios are optimized for both towing and everyday driving, providing a balance of power and efficiency.
Driving Characteristics
The 397 horsepower and 765 lb-ft of torque produced by the Duramax engine and Allison transmission combination deliver impressive performance. While not designed for outright speed, this Sierra 3500HD excels at moving substantial loads. Acceleration is brisk, even with a trailer attached, and the transmission shifts smoothly and decisively. The Allison transmission’s programming prioritizes smooth operation and minimizes gear hunting. Compared to the gasoline-powered versions of the Sierra 3500HD, the diesel offers significantly more low-end torque, making it ideal for towing heavy trailers up steep grades. Compared to other diesel trims, like those with manual transmissions, the automatic provides a more relaxed driving experience, particularly in stop-and-go traffic. However, the manual transmission variants generally offer slightly better fuel economy.
Equipment & Trim Levels
The 2011-2013 Sierra 3500HD Extended Cab Long Box with the Duramax diesel typically came standard with features like air conditioning, power windows and locks, a tilt steering wheel, and a basic audio system. Higher trim levels, such as the SLT and Denali, added features like leather upholstery, heated seats, a premium audio system, navigation, and a rearview camera. Optional extras included a trailer brake controller, a fifth-wheel hitch, and various appearance packages. The interior design was functional and durable, with a focus on providing a comfortable and productive work environment.
Chassis & Braking
The Sierra 3500HD’s chassis is built for heavy-duty use. It features a fully boxed frame constructed from high-strength steel. The front suspension utilizes an independent torsion bar setup with double wishbone control arms, 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 at all four corners, providing ample stopping power even when towing. An Anti-lock Braking System (ABS) is standard. The rear axle was typically a single rear wheel (SRW) configuration, though dual rear wheel (DRW) options were available for even greater towing capacity.
Market Reception & Comparison
The 2011-2013 GMC Sierra 3500HD with the Duramax diesel received generally positive reviews from automotive critics. It was praised for its powerful engine, smooth transmission, and comfortable ride. Fuel economy, while not exceptional, was considered reasonable for a heavy-duty truck of its size and capability. Compared to the gasoline-powered Sierra 3500HD models, the diesel offered significantly better fuel economy when towing. Reliability was generally good, although some owners reported issues with the diesel particulate filter (DPF) system.
Legacy
The Duramax LML engine and Allison transmission combination have proven to be exceptionally durable and reliable. Well-maintained examples of the 2011-2013 Sierra 3500HD continue to command strong resale values in the used car market. The powertrain is known for its longevity, with many trucks exceeding 200,000 miles without major issues. The GMT900 platform, and this variant in particular, established the Sierra 3500HD as a formidable competitor in the heavy-duty truck segment, solidifying GMC’s reputation for building tough and capable work trucks.
