The GMC Sierra 2500HD IV (GMTK2XX) Double Cab Long Box, equipped with the 6.6-liter Duramax Turbo Diesel V8 engine producing 397 horsepower and paired with a 6-speed Allison automatic transmission, represents a significant iteration in GMC’s heavy-duty truck lineage. Produced from 2015 to 2017, this configuration occupied a premium position within the Sierra 2500HD lineup, targeting buyers requiring substantial towing and hauling capabilities without sacrificing everyday usability. It built upon the foundation of the GMTK2XX platform, offering enhanced refinement and technology compared to its predecessors.
Technical Specifications
| Brand | GMC |
| Model | Sierra 2500HD |
| Generation | Sierra 2500HD IV (GMTK2XX) Double Cab Long Box |
| Type (Engine) | 6.6 Duramax TD V8 (397 Hp) 4WD Automatic |
| Start of production | 2015 |
| End of production | 2017 |
| Powertrain Architecture | Internal Combustion engine |
| Body type | Pick-up |
| Seats | 5 |
| Doors | 4 |
| Fuel Type | Diesel |
| Weight-to-power ratio | 8.2 kg/Hp, 121.7 Hp/tonne |
| Weight-to-torque ratio | 3.1 kg/Nm, 317.9 Nm/tonne |
| Power | 397 Hp @ 3000 rpm |
| Power per litre | 60.1 Hp/l |
| Torque | 1037 Nm @ 1600 rpm (764.85 lb.-ft. @ 1600 rpm) |
| Engine layout | Front, Longitudinal |
| Engine Model/Code | Duramax LML |
| Engine displacement | 6604 cm3 (403 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 | 22.4 l (23.67 US qt | 19.71 UK qt) |
| Kerb Weight | 3262 kg (7191.48 lbs.) |
| Max. weight | 4536 kg (10000.17 lbs.) |
| Max load | 1274 kg (2808.69 lbs.) |
| Trunk (boot) space – maximum | 2162 l (76.35 cu. ft.) |
| Fuel tank capacity | 136 l (35.93 US gal | 29.92 UK gal) |
| Permitted trailer load with brakes (12%) | 6577 kg (14499.8 lbs.) |
| Length | 6322 mm (248.9 in.) |
| Width including mirrors | 2045 mm (80.51 in.) |
| Height | 1977 mm (77.83 in.) |
| Wheelbase | 4017 mm (158.15 in.) |
| Front track | 1748 mm (68.82 in.) |
| Rear (Back) track | 1709 mm (67.28 in.) |
| Ride height (ground clearance) | 214 mm (8.43 in.) |
| Minimum turning circle (turning diameter) | 16.2 m (53.15 ft.) |
| Drivetrain Architecture | All wheel drive (4×4). Internal Combustion engine. The ICE drives the rear wheels permanently, and the front wheels are driven through an electrically or mechanically operated clutch if necessary. |
| Number of gears and type of gearbox | 6 gears, automatic transmission Allison |
| Front suspension | Coil spring, Double wishbone, Transverse stabilizer |
| Rear suspension | Rigid axle suspension, Leaf spring |
| Front brakes | Ventilated discs, 355×40 mm |
| Rear brakes | Ventilated discs, 360×34 mm |
| Assisting systems | ABS (Anti-lock braking system) |
| Steering type | Cone worm with recirculation balls |
| Tires size | 245/75 R17; 265/70 R17; 265/70 R18; 265/60 R20 |
| Wheel rims size | 7.5J x 17; 8J x 18; 8.5J x 20 |
Powertrain & Engine Architecture
The heart of this Sierra 2500HD variant is the Duramax LML 6.6-liter turbocharged diesel V8. This engine represents a significant evolution of the Duramax design, incorporating a redesigned combustion chamber, improved fuel injection system, and a more robust cooling system. The LML utilizes a common-rail direct injection system, delivering fuel at extremely high pressures for optimized combustion efficiency and reduced emissions. The engine also features a variable geometry turbocharger (VGT) which adjusts airflow to optimize performance across the RPM range. The overhead valve (OHV) valvetrain, while traditional, is known for its durability and simplicity. The Allison 1000 6-speed automatic transmission is specifically designed to handle the high torque output of the Duramax engine. It features a lock-up torque converter for improved fuel efficiency at highway speeds and heavy-duty internal components for reliability under demanding conditions.
Driving Characteristics
The 397 horsepower and 765 lb-ft of torque delivered by this powertrain configuration provide substantial performance for heavy-duty tasks. While not as overtly quick as gasoline-powered trucks, the Duramax offers a broad, flat torque curve, making it exceptionally capable for towing and hauling. The Allison transmission is well-matched to the engine, providing smooth and confident shifts. Compared to the 6.0L gasoline V8 option, the Duramax offers significantly more torque, making it the preferred choice for those regularly towing large trailers or heavy loads. The automatic transmission, while convenient, does result in a slight performance deficit compared to the available manual transmission, particularly in terms of responsiveness. However, the Allison’s programming prioritizes durability and smooth operation over outright acceleration.
Equipment & Trim Levels
The 6.6L Duramax with the Allison automatic was typically offered on higher trim levels of the Sierra 2500HD, such as the SLT and Denali. Standard features included air conditioning, power windows and locks, a premium audio system, and a rearview camera. The SLT trim added leather upholstery, heated front seats, and a navigation system. The Denali trim, the most luxurious offering, included unique exterior styling, upgraded interior materials, and additional convenience features. Optional extras included a sunroof, a premium Bose audio system, and advanced safety features like lane departure warning and forward collision alert.
Chassis & Braking
The Sierra 2500HD features a robust chassis designed to withstand the stresses of heavy-duty use. The front suspension utilizes coil springs and a double wishbone design, while the rear suspension employs a traditional leaf spring setup. This configuration provides a comfortable ride while maintaining excellent load-carrying capacity. Brakes consist of ventilated discs at all four corners, providing ample stopping power. The rear brakes are slightly smaller in diameter than the front brakes. An Anti-lock Braking System (ABS) is standard, enhancing safety during emergency braking situations. Steering is handled by a recirculating ball system, offering a direct and responsive feel.
Market Reception & Comparison
The 6.6L Duramax/Allison combination was widely praised by automotive critics for its exceptional towing and hauling capabilities, smooth operation, and relatively quiet operation for a diesel engine. Fuel economy, while not a primary concern for truck buyers, was respectable for a vehicle of this size and capability. Compared to the gasoline-powered Sierra 2500HD models, the Duramax offered significantly lower operating costs, particularly for those logging high mileage. Reliability was generally good, although some owners reported issues with the diesel particulate filter (DPF) system, a common concern with modern diesel engines.
Legacy
The GMTK2XX generation Sierra 2500HD with the Duramax 6.6L engine has proven to be a durable and reliable workhorse. On the used car market, these trucks command a premium price due to their reputation for longevity and capability. The LML Duramax engine is considered one of the most reliable diesel engines ever produced, and the Allison transmission is renowned for its durability. Properly maintained examples continue to provide years of dependable service, making them a popular choice for those seeking a capable and long-lasting heavy-duty truck.
