The GMC Sierra 2500HD III, produced between 2007 and 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. The Extended Cab Long Box configuration, paired with the 6.6-liter Duramax turbodiesel V8 engine producing 365 horsepower and 895 lb-ft of torque, and a six-speed Allison automatic transmission, was a popular choice for buyers needing substantial towing and hauling capacity. This particular variant occupied a sweet spot in the lineup, offering a powerful and efficient diesel option without the premium price tag of higher trim levels.
Technical Specifications
| Brand | GMC |
| Model | Sierra 2500HD |
| Generation | Sierra 2500HD III (GMT900) Extended Cab Long Box |
| Type (Engine) | 6.6 Duramax TD V8 (365 Hp) 4WD 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 | 8.3 kg/Hp, 120.2 Hp/tonne |
| Weight-to-torque ratio | 3.4 kg/Nm, 294.8 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 | 3036 kg, 6693.23 lbs. |
| Max. weight | 4173 kg, 9199.89 lbs. |
| Max load | 1137 kg, 2506.66 lbs. |
| Trunk (boot) space – maximum | 2138 l, 75.5 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 | 1930 mm, 75.98 in. |
| Wheelbase | 4001 mm, 157.52 in. |
| Front track | 1744 mm, 68.66 in. |
| Rear (Back) track | 1676 mm, 65.98 in. |
| Ride height (ground clearance) | 243 mm, 9.57 in. |
| Minimum turning circle (turning diameter) | 16 m, 52.49 ft. |
| Drivetrain Architecture | The Internal combustion engine (ICE) drives the rear wheels permanently, and the front wheels are driven through an electrically or mechanically operated clutch if necessary. |
| Drive wheel | All wheel drive (4×4) |
| 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 |
| Assisting systems | ABS (Anti-lock braking system) |
| Steering type | Cone worm with recirculation balls |
| Tires size | 245/75 R16; 265/75 R17 |
| Wheel rims size | 6.5J x 16; 7.5J x 17 |
INTRODUCTION
The 2007-2010 GMC Sierra 2500HD III, specifically the Extended Cab Long Box model equipped with the 6.6-liter Duramax diesel and Allison automatic, was a workhorse designed for demanding tasks. Built on the GMT900 platform, this truck represented a significant upgrade over its predecessor, offering increased payload and towing capacities, improved interior refinement, and enhanced safety features. The Extended Cab configuration provided additional passenger space, making it suitable for both work and family use, while the long bed maximized cargo-carrying versatility. This variant was positioned as a mid-range offering, balancing capability with affordability, appealing to a broad spectrum of buyers including contractors, farmers, and recreational users.
Powertrain & Engine Architecture
The heart of this Sierra 2500HD is the Duramax LMM 6.6-liter V8 diesel engine. This engine, a product of a joint venture between General Motors and Isuzu, featured a cast iron block, aluminum cylinder heads, and a robust crankshaft. The LMM engine utilized a common-rail direct fuel injection system, delivering fuel at extremely high pressures for optimized combustion. A variable geometry turbocharger (VGT) and an intercooler further enhanced performance and efficiency. The engine produced 365 horsepower at 3200 rpm and a substantial 895 lb-ft of torque at 1600 rpm. The Allison 1000 six-speed automatic transmission was specifically designed to handle the high torque output of the Duramax engine. This transmission featured electronically controlled shifts and a lock-up torque converter for improved fuel economy. The transmission’s gear ratios were optimized for both towing and everyday driving, providing a balance of power and efficiency.
Driving Characteristics
The 6.6-liter Duramax and Allison automatic combination delivered a driving experience characterized by immense pulling power and smooth, refined operation. While not as quick as gasoline-powered counterparts, the Sierra 2500HD offered effortless acceleration, even when heavily loaded. The Allison transmission shifted smoothly and predictably, providing seamless power delivery. The gear ratios were well-suited for towing, allowing the truck to maintain highway speeds with minimal effort. Compared to the 6.0-liter gasoline V8 option, the Duramax provided significantly more torque at lower engine speeds, making it ideal for hauling heavy loads. Compared to the manual transmission equipped Duramax models, the automatic offered greater convenience, particularly in stop-and-go traffic, but some performance was sacrificed. The automatic transmission did a good job of keeping the engine in its optimal power band, but lacked the direct control offered by a manual gearbox.
Equipment & Trim Levels
The Sierra 2500HD Extended Cab Long Box with the Duramax engine typically came standard with features such as air conditioning, power windows and locks, a CD player, and cloth upholstery. The Extended Cab provided a small rear seat suitable for occasional passengers. Optional extras included leather upholstery, a premium sound system, navigation, heated seats, and a power sunroof. Trim levels varied, but this configuration often fell into the SLE or SLT trims, offering a balance of features and value. The SLT trim added more luxurious appointments, such as chrome exterior trim and upgraded interior materials.
Chassis & Braking
The GMT900 chassis featured a fully boxed frame for increased rigidity and durability. The front suspension utilized an independent torsion bar setup with double wishbone control arms, providing a comfortable ride and precise handling. The rear suspension employed a traditional leaf spring design, optimized for 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 brakes were slightly smaller in diameter than the front brakes. The truck’s robust suspension and braking system were designed to handle the demands of heavy towing and hauling.
Market Reception & Comparison
The GMC Sierra 2500HD with the Duramax diesel engine was well-received by critics and consumers alike. It was praised for its powerful engine, smooth transmission, and comfortable ride. Fuel economy, while not exceptional, was competitive for a heavy-duty truck. Compared to the gasoline-powered Sierra 2500HD, the Duramax offered significantly better fuel economy and towing capacity. The Duramax/Allison combination was often considered superior to the equivalent Ford Super Duty and Ram 2500HD offerings in terms of refinement and overall driving experience. However, the Duramax engine was known to be more expensive to maintain than the gasoline engines, particularly regarding the fuel injection system and turbocharger.
Legacy
The Duramax LMM engine and Allison transmission combination proved to be exceptionally durable and reliable. Well-maintained examples of the 2007-2010 GMC Sierra 2500HD continue to command strong resale values in the used car market. The LMM Duramax is considered by many to be one of the most reliable Duramax engines ever produced. Common issues are typically related to routine maintenance items, such as fuel injectors and turbochargers, rather than catastrophic engine failures. The GMT900 platform established the Sierra 2500HD as a leading contender in the heavy-duty truck segment, setting the stage for future generations of GMC’s work trucks.
