The GMC Sierra 2500HD IV (GMTK2XX) Crew 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 was part of the fourth generation of the Sierra HD series, built on the GMTK2XX platform, which brought improvements in ride quality, interior refinement, and technology compared to its predecessors.
Technical Specifications
| Brand | GMC |
| Model | Sierra 2500HD |
| Generation | Sierra 2500HD IV (GMTK2XX) Crew Cab Long Box |
| Type (Engine) | 6.6 Duramax TD V8 (397 Hp) 4WD Automatic |
| Start of production | 2015 year |
| End of production | 2017 year |
| Powertrain Architecture | Internal Combustion engine |
| Body type | Pick-up |
| Seats | 5 |
| Doors | 4 |
| Fuel Type | Diesel |
| Weight-to-power ratio | 8.6 kg/Hp, 115.9 Hp/tonne |
| Weight-to-torque ratio | 3.3 kg/Nm, 302.8 Nm/tonne |
| Power | 397 Hp @ 3000 rpm |
| Power per litre | 60.2 Hp/l |
| Torque | 1037 Nm @ 1600 rpm, 764.85 lb.-ft. @ 1600 rpm |
| Engine layout | Front, Longitudinal |
| Engine Model/Code | Duramax LML |
| 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 | 16:1 |
| 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 | 3425 kg, 7550.83 lbs. |
| Max. weight | 4536 kg, 10000.17 lbs. |
| Max load | 1111 kg, 2449.34 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 | 6563 mm, 258.39 in. |
| Width including mirrors | 2045 mm, 80.51 in. |
| Height | 1980 mm, 77.95 in. |
| Wheelbase | 4258 mm, 167.64 in. |
| Front track | 1748 mm, 68.82 in. |
| Rear (Back) track | 1709 mm, 67.28 in. |
| Ride height (ground clearance) | 208 mm, 8.19 in. |
| Minimum turning circle (turning diameter) | 17.1 m, 56.1 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 | Coil spring, Double wishbone, Transverse stabilizer |
| Rear suspension | Rigid axle suspension, Leaf spring |
| Front brakes | Ventilated discs, 350×40 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 is the Duramax LML 6.6-liter turbocharged diesel V8. This engine represents a significant evolution of the Duramax design, incorporating a robust cast iron block, aluminum cylinder heads, and a variable geometry turbocharger. The LML engine utilizes a high-pressure common rail fuel injection system, delivering fuel at over 30,000 psi for optimized combustion and reduced emissions. The overhead valve (OHV) valvetrain, while traditional, is known for its durability and simplicity. The engine’s 16:1 compression ratio contributes to its efficiency and power output. 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 economy and a heavy-duty cooling system to prevent overheating under demanding conditions. The transmission’s gear ratios are optimized for both towing and everyday driving, providing a balance between power and efficiency.
Driving Characteristics
The 6.6-liter Duramax with the Allison automatic delivers a commanding driving experience. While not as quick off the line as gasoline-powered trucks, the immense 1037 Nm (764.85 lb-ft) of torque provides effortless acceleration, especially when towing. The Allison transmission shifts smoothly and decisively, making it easy to maintain control even with a heavy load. Compared to the 6.0-liter gasoline V8 option, the Duramax offers significantly more low-end torque and superior towing capacity. The automatic transmission, while convenient, does result in a slight reduction in fuel economy compared to the available manual transmission. However, the convenience and ease of use make it a popular choice for many drivers. The long wheelbase and heavy-duty suspension contribute to a comfortable ride, even when unladen.
Equipment & Trim Levels
The Crew Cab Long Box configuration, combined with the Duramax engine, typically came in higher trim levels such as SLE, SLT, and Denali. Standard features included air conditioning, power windows and locks, a rearview camera, and a basic infotainment system. SLT trims added leather upholstery, heated front seats, a premium sound system, and additional chrome exterior trim. The Denali trim, the most luxurious option, featured unique styling cues, upgraded interior materials, and advanced technology features like a navigation system and a head-up display. Optional extras included a sunroof, a rear-seat entertainment system, and various towing packages.
Chassis & Braking
The GMTK2XX platform features a fully boxed frame for increased rigidity and strength. The front suspension utilizes coil springs and a double wishbone design, while the rear suspension employs a traditional leaf spring setup. This combination provides a good balance between ride comfort and load-carrying capacity. Braking duties are handled by ventilated discs at all four corners, with larger rotors and more robust calipers compared to lighter-duty trucks. An Anti-lock Braking System (ABS) is standard. The rear axle ratios varied depending on the specific configuration, influencing both towing capacity and fuel economy.
Market Reception & Comparison
The 2015-2017 GMC Sierra 2500HD with the Duramax diesel and Allison automatic was well-received by critics and consumers alike. It was praised for its powerful engine, smooth transmission, comfortable ride, and refined interior. Compared to competitors like the Ford Super Duty and Ram 2500, the Sierra 2500HD offered a compelling combination of performance, luxury, and technology. Fuel economy, while respectable for a heavy-duty truck, was not its strongest suit, but the Duramax engine’s longevity and reliability helped offset this concern. The Duramax/Allison combination generally proved more reliable than the gasoline engine options, although diesel engines require more diligent maintenance.
Legacy
The Duramax LML engine paired with the Allison transmission has proven to be a remarkably durable and reliable powertrain. These trucks continue to hold their value well in the used car market, particularly those that have been well-maintained. The LML Duramax is known for its ability to withstand high mileage and heavy use. Common maintenance items include regular oil changes, fuel filter replacements, and coolant flushes. While diesel engines can be more expensive to repair than gasoline engines, the longevity and towing capacity of the Sierra 2500HD with this powertrain make it a popular choice for those who need a capable and dependable heavy-duty truck.
