The GMC Sierra 3500HD III, specifically the Extended Cab Long Box configuration with the 6.6-liter Duramax Turbo Diesel V8 engine and four-wheel drive, 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 at buyers needing substantial towing and hauling capacity without sacrificing some degree of interior space and comfort. It occupied a sweet spot in the Sierra 3500HD lineup, offering a powerful diesel option paired with the convenience of an automatic transmission, appealing to both commercial users and private owners engaged in demanding tasks.
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) 4WD 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 | 8.1 kg/Hp, 124.1 Hp/tonne |
| Weight-to-torque ratio | 3.1 kg/Nm, 324.3 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 | 3198 kg (7050.38 lbs.) |
| Max. weight | 4990 kg (11001.07 lbs.) |
| Max load | 1792 kg (3950.68 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) | 213 mm (8.39 in.) |
| Minimum turning circle (turning diameter) | 15.8 m (51.84 ft.) |
| Drivetrain Architecture | All wheel drive (4×4). ICE drives rear wheels, front wheels engaged via clutch. |
| 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 |
| Assisting systems | ABS (Anti-lock braking system) |
| Steering type | Cone worm with recirculation balls |
| Tires size | 235/80 R17; 265/70 R18 |
Powertrain & Engine Architecture
The heart of this Sierra 3500HD is the Duramax LML 6.6-liter turbocharged diesel V8. This engine, a significant evolution of the Duramax design, featured a cast iron block and aluminum cylinder heads. The LML introduced a revised combustion chamber and a cooled exhaust gas recirculation (EGR) system to further reduce emissions. The engine utilized a high-pressure common rail direct fuel injection system, delivering fuel at extremely high pressures for optimized combustion. The overhead valve (OHV) valvetrain, while traditional, was robust and reliable. The Allison 6-speed automatic transmission was specifically calibrated to handle the immense torque output of the Duramax engine. This transmission was known for its durability and smooth shifting, crucial for heavy-duty applications. The transmission featured a lock-up torque converter to improve fuel efficiency at highway speeds.
Driving Characteristics
The 397 horsepower and 765 lb-ft of torque delivered by the Duramax engine in this configuration provided substantial pulling power. While not as quick as lighter-duty pickups, the Sierra 3500HD with this powertrain offered confident acceleration even when heavily loaded. The Allison automatic transmission, while not offering the driver ultimate control, provided smooth and predictable gear changes. The gear ratios were optimized for towing, with lower gears providing ample torque for starting and climbing hills. Compared to the 6.0L gasoline V8 option, the Duramax offered significantly more torque at lower RPMs, making it ideal for demanding tasks. Compared to the 1.9 TDI or VR6 options found in European equivalents, the Duramax offered a substantially higher power output and towing capacity, albeit with a greater overall weight.
Equipment & Trim Levels
The Extended Cab configuration provided additional rear seating and storage space compared to the regular cab models. Standard features typically included 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 chrome exterior trim. Optional extras included a trailer brake controller, a fifth-wheel hitch, and various off-road packages. The interior design was functional and durable, prioritizing practicality over luxury.
Chassis & Braking
The Sierra 3500HD featured a heavy-duty chassis designed to withstand the stresses of heavy towing and hauling. The front suspension utilized an independent torsion bar setup with double wishbones, while the rear suspension employed a traditional leaf spring design. This combination provided a comfortable ride while maintaining excellent load-carrying capacity. Braking duties were handled by ventilated disc brakes on all four wheels, providing ample stopping power. An Anti-lock Braking System (ABS) was standard. The rear axle was a solid axle, contributing to the truck’s robust towing capabilities.
Market Reception & Comparison
The 2011-2013 GMC Sierra 3500HD with the Duramax diesel and Allison automatic was generally well-received by critics and consumers. It was praised for its powerful engine, smooth transmission, and impressive towing capacity. Fuel economy, while not exceptional, was respectable for a heavy-duty truck. Compared to the gasoline-powered Sierra 3500HD models, the Duramax offered significantly better fuel economy and torque. 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 proved to be exceptionally durable, and well-maintained examples of this Sierra 3500HD continue to be highly sought after in the used truck market. The LML Duramax is considered one of the most reliable diesel engines ever produced. These trucks are often used for demanding tasks such as towing large trailers, hauling heavy loads, and working in construction or agriculture. The combination of power, reliability, and capability has cemented the Sierra 3500HD’s reputation as a workhorse among heavy-duty pickup trucks.
