The GMC Sierra 3500HD III, specifically the Extended Cab Long Box configuration powered by the 6.6-liter Duramax turbodiesel V8 producing 365 horsepower, represents a significant iteration in GMC’s heavy-duty truck lineage. Produced from 2007 through 2010 as part of the GMT900 platform generation, this variant was aimed squarely at buyers needing substantial towing and hauling capacity without sacrificing some degree of interior space and comfort. It occupied a sweet spot in the 3500HD lineup, offering a powerful diesel engine paired with the convenience of an automatic transmission, making it a popular choice for both commercial and private users.
Technical Specifications
| Brand | GMC |
| Model | Sierra 3500HD |
| Generation | Sierra 3500HD III (GMT900) Extended Cab Long Box |
| Type (Engine) | 6.6 Duramax TD V8 (365 Hp) SRW Automatic |
| Start of production | 2007 |
| End of production | 2010 |
| Powertrain Architecture | Internal Combustion engine |
| Body type | Pick-up |
| Seats | 5 |
| Doors | 2 |
| Fuel Type | Diesel |
| Weight-to-power ratio | 8.2 kg/Hp, 122.4 Hp/tonne |
| Weight-to-torque ratio | 3.3 kg/Nm, 300 Nm/tonne |
| Power | 365 Hp @ 3200 rpm |
| Power per litre | 55.3 Hp/l |
| Torque | 895 Nm @ 1600 rpm (660.12 lb.-ft. @ 1600 rpm) |
| Maximum engine speed | 3450 rpm |
| Engine layout | Front, Longitudinal |
| Engine Model/Code | Duramax LMM |
| 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 | 16.8:1 |
| 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 | 2983 kg (6576.39 lbs.) |
| Max. weight | 4400 kg (9700.34 lbs.) |
| Max load | 1417 kg (3123.95 lbs.) |
| Trunk (boot) space – maximum | 2170 l (76.63 cu. ft.) |
| Fuel tank capacity | 129 l (34.08 US gal | 28.38 UK gal) |
| Permitted trailer load with brakes (12%) | 5897 kg (13000.66 lbs.) |
| Length | 6330 mm (249.21 in.) |
| Width including mirrors | 2436 mm (95.91 in.) |
| Height | 1931 mm (76.02 in.) |
| Wheelbase | 4001 mm (157.52 in.) |
| Front track | 1742 mm (68.58 in.) |
| Rear (Back) track | 1676 mm (65.98 in.) |
| Ride height (ground clearance) | 233 mm (9.17 in.) |
| Minimum turning circle (turning diameter) | 16 m (52.49 ft.) |
| Drivetrain Architecture | Rear wheel drive |
| Number of gears and type of gearbox | 6 gears, automatic transmission Allison |
Powertrain & Engine Architecture
The heart of this Sierra 3500HD is the Duramax LMM 6.6-liter V8 diesel engine. This engine, a product of a joint venture between General Motors and Isuzu, represented a significant upgrade over its predecessor, the LBZ. The LMM featured a redesigned combustion chamber and improved fuel injection system, contributing to increased power and reduced emissions. The engine utilizes a common-rail direct injection system, delivering fuel at extremely high pressures for optimized combustion. The Duramax engine employs an overhead valve (OHV) valvetrain, a design choice favored for its durability and relatively compact size. A variable geometry turbocharger with an intercooler further enhances performance by providing increased air intake and reducing intake air temperature. The Allison 1000 six-speed automatic transmission was the standard pairing for this engine. Known for its robustness and smooth shifting, the Allison transmission was specifically designed to handle the high torque output of the Duramax diesel, making it ideal for heavy-duty applications.
Driving Characteristics
The 365 horsepower and 895 lb-ft of torque delivered by the Duramax/Allison combination translated into impressive real-world performance. While not a sports truck, the Sierra 3500HD offered ample power for accelerating with a load or maintaining highway speeds. The automatic transmission, with its six forward gears, provided a good balance between fuel efficiency and responsiveness. Compared to the manual transmission options available on other 3500HD trims, the automatic prioritized ease of use, particularly in stop-and-go traffic or when towing. The gear ratios were optimized for pulling power, with a relatively low first gear for launching heavy loads. However, this also meant that the engine could feel somewhat strained at higher speeds without a load.
Equipment & Trim Levels
The Extended Cab configuration provided additional rear seating and storage space compared to the standard cab models. Standard features typically included air conditioning, power windows and locks, an AM/FM stereo, and a basic level of interior trim. The Long Box provided a substantial cargo bed, ideal for hauling large items. Optional extras included leather upholstery, a premium sound system, navigation, and various towing packages. Trim levels varied, but this configuration often fell into the mid-range offerings, providing a good balance of features and affordability.
Chassis & Braking
The Sierra 3500HD featured a robust chassis designed to withstand the stresses of heavy-duty use. The front suspension utilized an independent torsion bar setup with double wishbones, providing a comfortable ride while maintaining stability. The rear suspension employed a traditional leaf spring rigid axle configuration, known for its durability and load-carrying capacity. Braking duties were handled by ventilated discs at all four corners, providing ample stopping power. An Anti-lock Braking System (ABS) was standard, enhancing safety during emergency braking situations.
Market Reception & Comparison
The 6.6L Duramax/Allison combination in the Sierra 3500HD was generally well-received by critics and consumers alike. It was praised for its strong towing capacity, smooth ride, and relatively quiet operation for a diesel engine. Compared to the gasoline-powered options, the Duramax offered significantly better fuel economy, especially when towing. However, the diesel engine came with a higher initial cost and potentially higher maintenance expenses. Compared to the 1.9 TDI or VR6 options found in European trucks, the Duramax offered a substantially higher towing capacity and torque output, but at the expense of fuel efficiency and refinement.
Legacy
The Duramax LMM engine, paired with the Allison transmission, has proven to be a remarkably durable powertrain. Many examples of these trucks are still in service today, having accumulated hundreds of thousands of miles. While some common issues, such as injector failures and turbocharger problems, have been reported, these are often manageable with proper maintenance. On the used car market, these Sierra 3500HDs continue to hold their value, particularly those that have been well-maintained and have a documented service history. They remain a popular choice for buyers seeking a capable and reliable heavy-duty truck.
