The GMC Sierra 2500HD III, specifically the Crew 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, comfortable, and powerful work trucks. It occupied a premium position within the 2500HD lineup, offering a substantial increase in towing and hauling capacity compared to lighter-duty Sierra models, and a more refined experience than the base engine options.
Technical Specifications
| Brand | GMC |
| Model | Sierra 2500HD |
| Generation | Sierra 2500HD III (GMT900, facelift 2011) Crew 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 | 4 |
| Fuel Type | Diesel |
| Weight-to-power ratio | 8.1 kg/Hp, 123.8 Hp/tonne |
| Weight-to-torque ratio | 3.1 kg/Nm, 323.5 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 | 3206 kg (7068.02 lbs.) |
| Max. weight | 4536 kg (10000.17 lbs.) |
| Max load | 1330 kg (2932.15 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 | 6581 mm (259.09 in.) |
| Width including mirrors | 2032 mm (80 in.) |
| Height | 1984 mm (78.11 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) | 211 mm (8.31 in.) |
| Minimum turning circle (turning diameter) | 16.7 m (54.79 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 diesel engine. This engine represents a significant evolution of the Duramax design, incorporating a redesigned combustion chamber, improved fuel injection system, and a more efficient turbocharger. The LML utilizes a common-rail direct injection system, delivering fuel at extremely high pressures for optimized combustion. The engine also features a variable geometry turbocharger (VGT) which adjusts airflow based on engine speed and load, improving both power and fuel efficiency. The overhead valve (OHV) valvetrain, while traditional, is robust and well-suited to the demands of a heavy-duty truck. Paired with the engine is the Allison 1000 six-speed automatic transmission, renowned for its durability and ability to handle high torque outputs. This transmission features a lock-up torque converter for improved fuel economy at highway speeds and provides smooth, responsive shifting.
Driving Characteristics
The 397 horsepower and 765 lb-ft of torque produced by this combination translate into impressive real-world performance. While not designed for outright speed, the Sierra 2500HD with this powertrain excels at towing and hauling heavy loads. The Allison transmission’s gear ratios are optimized for pulling power, providing ample torque even at lower engine speeds. Compared to the 6.0L gasoline V8 option, the Duramax diesel offers significantly more torque, making it the preferred choice for those regularly engaging in demanding work. While a manual transmission was available on other 2500HD trims, the Allison automatic provided a more convenient and comfortable driving experience, particularly in stop-and-go traffic or when navigating challenging terrain. The ride quality, while firm due to the heavy-duty suspension, is surprisingly compliant for a truck of this size.
Equipment & Trim Levels
The Crew Cab Long Box configuration, combined with the Duramax diesel, typically came in higher trim levels such as SLT or Denali. Standard features included air conditioning, power windows and locks, a premium audio system, and cloth upholstery. The SLT trim added features like leather upholstery, heated front seats, and a navigation system. The Denali trim, the most luxurious option, included unique exterior styling, upgraded interior materials, and additional convenience features. Optional extras included a sunroof, rear-seat entertainment system, and various towing packages.
Chassis & Braking
The Sierra 2500HD’s chassis is built around a fully boxed frame, providing a solid foundation for heavy-duty use. The front suspension utilizes an independent torsion bar setup with double wishbones, while the rear suspension employs a traditional leaf spring design. This configuration provides a good balance of ride comfort and load-carrying capacity. Braking duties are handled by ventilated disc brakes on all four wheels, with ABS standard. The rear brakes are slightly larger than the front, reflecting the greater weight distribution at the rear of the truck when loaded.
Market Reception & Comparison
The Duramax diesel-powered Sierra 2500HD was widely praised by automotive critics for its powerful engine, smooth transmission, and comfortable ride. It consistently outperformed its gasoline-powered counterparts in towing and hauling tests. Fuel economy, while not exceptional, was respectable for a truck of this size, particularly when considering its capabilities. Compared to the Ford Super Duty and Ram 2500, the Sierra 2500HD offered a competitive combination of features, performance, and reliability. Some critics noted the higher price tag of the Duramax diesel option, but most agreed that the added cost was justified for those who needed the extra power and capability.
Legacy
The GMT900-generation Sierra 2500HD, and particularly the 6.6L Duramax/Allison combination, has proven to be a remarkably durable and reliable truck. These trucks continue to be popular on the used car market, commanding relatively high prices due to their reputation for longevity and capability. The LML Duramax engine is known for its robustness, and with proper maintenance, can easily exceed 200,000 miles. The Allison transmission is equally reliable, further contributing to the truck’s long-term value. The 2011-2014 Sierra 2500HD represents a sweet spot in the heavy-duty truck market, offering a blend of modern features, proven reliability, and impressive performance.
