The GMC Sierra 2500HD III (GMT900) Extended Cab Long Box, featuring the 6.6-liter Duramax Turbo Diesel V8 engine producing 365 horsepower and paired with an Allison automatic transmission, was a significant model in GMC’s heavy-duty pickup truck lineup from 2007 to 2010. This configuration struck a balance for buyers requiring substantial towing and hauling capabilities without the expense of the top-tier Denali trims. It positioned itself between base work truck models and the more luxurious Denali, attracting a diverse customer base including contractors, ranchers, and individuals needing a robust truck for personal use. The GMT900 platform, introduced in 2007, brought substantial improvements to the Sierra HD series, emphasizing increased capability and refinement.
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) 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 kg/Hp, 125.2 Hp/tonne |
| Weight-to-torque ratio | 3.3 kg/Nm, 307 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 | 2915 kg (6426.47 lbs.) |
| Max. weight | 4173 kg (9199.89 lbs.) |
| Max load | 1258 kg (2773.42 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 | 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 | 245/75 R16; 265/75 R17 |
| Wheel rims size | 6.5J x 16; 7.5J x 17 |
The core of this Sierra 2500HD was the Duramax LMM 6.6-liter V8 diesel engine. This engine represented a significant advancement in Duramax technology, incorporating a common-rail fuel injection system capable of extremely high pressures for enhanced combustion efficiency and reduced emissions. The LMM featured a variable geometry turbocharger (VGT) which optimized boost across the engine’s RPM range, contributing to both power and fuel economy. The engine utilized an overhead valve (OHV) valvetrain, a design choice favored for its durability and relatively compact size. The Allison 1000 six-speed automatic transmission was specifically engineered to manage the substantial torque output of the Duramax engine. This transmission featured a lock-up torque converter for improved efficiency at highway speeds and was renowned for its robustness and reliability.
Driving Characteristics
The 365 horsepower and 895 lb-ft of torque generated by the Duramax/Allison pairing delivered substantial performance, even when towing substantial loads. While not as quick as gasoline-powered trucks, the Sierra 2500HD offered impressive acceleration and effortless cruising. The Allison transmission was well-tuned for towing, with gear ratios optimized to keep the engine within its peak torque range. Compared to the 4.8L or 6.0L gasoline engine options available in lighter-duty Sierras, this diesel offered a fundamentally different driving experience – one centered on raw pulling power and durability rather than nimble handling. The automatic transmission, while convenient, did result in a slight decrease in fuel economy compared to the available manual transmission, but the trade-off was often considered worthwhile for ease of use, particularly when towing.
Equipment & Trim Levels
The Extended Cab Long Box configuration provided a practical balance of passenger space and cargo capacity. Standard features on this trim typically included air conditioning, power windows and locks, a basic audio system, and vinyl upholstery. Popular optional extras included leather upholstery, a premium audio system, navigation, and various towing packages. The interior design was functional and durable, prioritizing practicality over luxury. The extended cab offered additional seating for passengers, although the rear seats were best suited for occasional use.
Chassis & Braking
The Sierra 2500HD featured a heavy-duty chassis designed to withstand the stresses of frequent towing and hauling. 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 design, known for its durability and load-carrying capacity. Braking was provided by ventilated disc brakes on all four wheels, offering ample stopping power even with heavy loads. An Anti-lock Braking System (ABS) was standard, enhancing safety and control.
Market Reception & Comparison
The 6.6L Duramax/Allison combination received widespread acclaim from automotive critics and truck buyers. It consistently ranked among the best heavy-duty diesel powertrains available. Compared to the gasoline engine options, the Duramax offered significantly more torque, making it the preferred choice for those who regularly towed or hauled heavy loads. While the initial cost was higher, the Duramax’s superior fuel economy and longevity often resulted in lower overall operating costs. Compared to other diesel-powered trucks in the same class, such as the Ford Super Duty and Ram 2500, the Sierra 2500HD was often praised for its refined ride quality and comfortable interior.
Legacy
The Duramax LMM engine and Allison 1000 transmission have proven to be remarkably durable and reliable powertrains. Well-maintained examples continue to provide years of service. In the used car market, Sierra 2500HDs equipped with this powertrain command a premium price due to their reputation for longevity and capability. Common maintenance items include regular oil changes, fuel filter replacements, and occasional injector cleaning. While some early LMM engines experienced issues with the fuel injectors, these problems were largely addressed through updated parts and software calibrations. Overall, the GMC Sierra 2500HD III (GMT900) with the 6.6 Duramax and Allison automatic remains a highly desirable truck for those needing a powerful and dependable workhorse.
