The GMC Sierra 3500HD III, produced from 2007 to 2010, represented a significant evolution in GMC’s heavy-duty truck lineup. This generation, designated GMT900, offered a blend of increased capability, refined comfort, and updated technology. Within the 3500HD range, the Extended Cab Long Box configuration with the 6.6-liter Duramax Turbo Diesel V8 engine paired with an Allison automatic transmission was a popular choice for buyers needing substantial towing and hauling capacity without sacrificing everyday usability. This particular variant occupied a sweet spot in the lineup, offering a substantial step up in performance from the gasoline engine options while remaining more accessible than the fully-loaded premium trims.
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) DRW 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 | 8.6 kg/Hp, 116.3 Hp/tonne |
| Weight-to-torque ratio | 3.5 kg/Nm, 285.1 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 | 3139 kg (6920.31 lbs.) |
| Max. weight | 5171 kg (11400.1 lbs.) |
| Max load | 2032 kg (4479.79 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 | 1897 mm (74.69 in.) |
| Ride height (ground clearance) | 233 mm (9.17 in.) |
| 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, 325×38 mm |
| Rear brakes | Ventilated discs, 325×30 mm |
| Tires size | 265/75 R16; 265/70 R17; 225/75 R17 |
| Wheel rims size | 6.5J x 17; 7J x 16; 7.5J x 17 |
INTRODUCTION
The GMC Sierra 3500HD III, specifically the Extended Cab Long Box configuration with the 6.6L Duramax diesel and Allison automatic, was a workhorse designed for serious towing and hauling. Introduced in 2007 as part of the GMT900 series, it represented a significant upgrade over the previous generation. The “III” designation signified a major refresh, incorporating improvements in styling, interior refinement, and powertrain technology. This truck was primarily aimed at commercial users, farmers, and individuals who regularly required the capability to tow large trailers, such as fifth-wheels, gooseneck trailers, or heavy equipment. The Extended Cab provided additional passenger space compared to the regular cab models, while the Long Box offered a substantial cargo bed for hauling materials.
Powertrain & Engine Architecture
The heart of this Sierra 3500HD was the Duramax LMM 6.6-liter turbocharged diesel V8 engine. This engine, a product of a joint venture between General Motors and Isuzu, was renowned for its durability and torque output. The LMM featured a cast iron block, aluminum cylinder heads, and a robust crankshaft designed to withstand the stresses of heavy-duty operation. The engine utilized a high-pressure common rail fuel injection system, delivering fuel directly into the cylinders at extremely high pressures for optimized combustion. The turbocharger, coupled with an intercooler, increased air intake density, boosting power and efficiency. The valvetrain employed an overhead valve (OHV) configuration, known for its simplicity and reliability. Paired with the Duramax was the Allison 1000 six-speed automatic transmission, a heavy-duty unit specifically engineered to handle the engine’s substantial torque. The Allison transmission featured a lock-up torque converter for improved fuel economy at highway speeds and provided smooth, reliable shifting even under heavy loads.
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 even when towing. The Allison transmission’s six gears allowed the engine to operate within its optimal power band, maximizing efficiency and minimizing strain. Compared to the gasoline engine options, the diesel provided significantly more low-end torque, making it ideal for towing and hauling. The automatic transmission, while convenient, did result in a slight performance deficit compared to the available manual transmission, which allowed for more direct control over gear selection. However, the Allison’s smooth shifting and reliability made it a popular choice for most buyers. The truck’s heavy-duty suspension and robust chassis provided a stable and controlled ride, even when fully loaded.
Equipment & Trim Levels
The Sierra 3500HD offered a range of trim levels, with the specific features varying depending on the chosen configuration. The base models typically included features such as air conditioning, power windows and locks, 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, and navigation. The Extended Cab configuration provided a second row of seating, offering increased passenger capacity. Optional extras included a sunroof, upgraded wheels, and various towing packages. Interior upholstery typically consisted of cloth or vinyl, with leather being available on higher trims. The dashboard layout was functional and straightforward, prioritizing ease of use over flashy aesthetics.
Chassis & Braking
The Sierra 3500HD’s chassis was designed for durability and strength. It featured a fully boxed frame constructed from high-strength steel, providing a solid foundation for the truck’s heavy-duty components. The front suspension utilized an independent torsion bar setup with double wishbone control arms, offering a comfortable ride and good handling. The rear suspension employed a traditional leaf spring design, providing excellent load-carrying capacity. Braking duties were handled by ventilated disc brakes on all four wheels, providing ample stopping power even when towing. An Anti-lock Braking System (ABS) was standard, enhancing safety and control during emergency braking situations. The rear axle ratio was optimized for towing, providing a balance between power and fuel economy.
Market Reception & Comparison
The GMC Sierra 3500HD with the Duramax diesel and Allison automatic received generally positive reviews from critics and consumers alike. It was praised for its powerful engine, smooth transmission, and impressive towing capacity. Compared to the gasoline engine options, the diesel offered superior fuel economy and significantly more torque. However, the diesel engine did come with a higher initial cost and potentially higher maintenance expenses. Compared to competitors like the Ford Super Duty and Ram Heavy Duty, the Sierra 3500HD was often seen as offering a good balance of capability, comfort, and value. The Duramax/Allison combination was particularly well-regarded for its reliability and longevity.
Legacy
The GMT900-generation Sierra 3500HD, and particularly the 6.6L Duramax/Allison combination, has proven to be a durable and reliable truck. These trucks remain popular in the used car market today, commanding relatively high prices due to their reputation for longevity and capability. The LMM Duramax engine is known for its robustness, and with proper maintenance, can easily exceed 200,000 miles. The Allison transmission is also highly regarded for its durability. However, potential buyers should be aware of potential maintenance issues associated with the diesel engine, such as injector problems and turbocharger wear. Overall, the GMC Sierra 3500HD III with the 6.6L Duramax and Allison automatic remains a highly capable and respected work truck, continuing to serve owners well years after its production ended.
