The GMC Sierra 2500HD IV (GMTK2XX) Regular Cab Long Box, equipped with the 6.6-liter Duramax Turbo Diesel V8 engine producing 397 horsepower and paired with an Allison automatic transmission, was a mainstay of GMC’s heavy-duty truck lineup from 2015 to 2017. This configuration represented a work-focused variant within the fourth generation of the Sierra 2500HD, designed for customers prioritizing towing capacity and durability over luxury appointments. It occupied a middle ground in the lineup, offering a significant step up in capability from the gasoline-powered engines while remaining more accessible than the fully-loaded Denali trims.
Technical Specifications
| Brand | GMC |
| Model | Sierra 2500HD |
| Generation | Sierra 2500HD IV (GMTK2XX) Regular Cab Long Box |
| Type (Engine) | 6.6 Duramax TD V8 (397 Hp) Automatic |
| Start of production | 2015 |
| End of production | 2017 |
| Powertrain Architecture | Internal Combustion engine |
| Body type | Pick-up |
| Seats | 2 |
| Doors | 2 |
| Fuel Type | Diesel |
| Weight-to-power ratio | 7.5 kg/Hp, 132.6 Hp/tonne |
| Weight-to-torque ratio | 2.9 kg/Nm, 346.5 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 | 2993 kg (6598.44 lbs.) |
| Max. weight | 4491 kg (9900.96 lbs.) |
| Max load | 1498 kg (3302.52 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 | 5699 mm (224.37 in.) |
| Width including mirrors | 2035 mm (80.12 in.) |
| Height | 1976 mm (77.8 in.) |
| Wheelbase | 3393 mm (133.58 in.) |
| Front track | 1748 mm (68.82 in.) |
| Rear (Back) track | 1709 mm (67.28 in.) |
| Ride height (ground clearance) | 231 mm (9.09 in.) |
| Minimum turning circle (turning diameter) | 14 m (45.93 ft.) |
| Drivetrain Architecture | The Internal combustion engine (ICE) drives the rear wheels of the vehicle. |
| Drive wheel | Rear wheel drive |
| 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 |
INTRODUCTION
The 2015-2017 GMC Sierra 2500HD Regular Cab Long Box with the 6.6L Duramax diesel and Allison automatic transmission was a key offering in GMC’s heavy-duty truck range. Built on the GMTK2XX platform, shared with the Chevrolet Silverado 2500HD, this truck was designed to meet the demands of commercial users and individuals needing substantial towing and hauling capabilities. The Regular Cab configuration, combined with the long bed, prioritized practicality and cargo space, making it a popular choice for fleet applications and those who frequently transport large items. This variant aimed to provide a balance between capability and affordability within the Sierra 2500HD lineup.
Powertrain & Engine Architecture
The heart of this Sierra 2500HD is the Duramax LML 6.6-liter turbocharged diesel V8. This engine is a testament to diesel engineering, featuring a cast iron block, aluminum cylinder heads, and a robust crankshaft. The LML generation of the Duramax introduced a revised turbocharger, improved fuel injection system, and enhanced glow plug technology for improved cold-start performance and reduced emissions. The engine utilizes a common-rail direct injection system, delivering fuel at extremely high pressures for efficient combustion. The overhead valve (OHV) valvetrain, while seemingly dated, proved reliable and contributed to the engine’s low-end torque characteristics. The Allison 1000 six-speed automatic transmission is specifically designed to handle the immense torque output of the Duramax engine. This transmission features a lock-up torque converter for improved fuel efficiency and a heavy-duty cooling system to prevent overheating under strenuous loads. The transmission’s gear ratios are optimized for both towing and everyday driving, providing a balance of power and efficiency.
Driving Characteristics
The 6.6L Duramax/Allison combination in the Sierra 2500HD delivers a driving experience focused on capability rather than outright speed. While 397 horsepower is respectable, the true strength of this powertrain lies in its 765 lb-ft of torque. This translates to effortless towing, even with substantial trailers. Acceleration is brisk, but the truck’s weight is always apparent. The Allison transmission shifts smoothly and decisively, providing ample power for merging and passing. Compared to the gasoline-powered Sierra 2500HD models, the diesel offers significantly more low-end grunt and a flatter torque curve. Compared to higher-output Duramax variants (available in later years), this version provides a more manageable power delivery, making it easier to control when towing heavy loads. The automatic transmission, while convenient, doesn’t offer the same level of control as a manual transmission, but it simplifies operation for drivers who spend long hours behind the wheel.
Equipment & Trim Levels
The Regular Cab Long Box 2500HD with the Duramax engine typically came with a fairly basic level of equipment, reflecting its work-oriented focus. Standard features included air conditioning, a basic audio system, vinyl upholstery, and manual windows and locks. However, options were available to enhance comfort and convenience. These included power windows and locks, a premium audio system, cruise control, and upgraded interior trim. The interior design was functional and durable, prioritizing practicality over luxury. The dashboard layout was straightforward, with easy-to-read gauges and logically placed controls. The long bed provided ample cargo space, and the truck could be equipped with various bed accessories, such as tie-down points and bed liners.
Chassis & Braking
The Sierra 2500HD’s chassis is built for heavy-duty use. It features a fully boxed frame constructed from high-strength steel, providing a solid foundation for the truck’s components. The front suspension utilizes coil springs and a double wishbone design, while the rear suspension employs leaf springs and a solid axle. This configuration provides a comfortable ride, even when the truck is heavily loaded. The braking system consists of ventilated disc brakes on all four wheels, providing ample stopping power. An Anti-lock Braking System (ABS) is standard, enhancing safety and control during emergency braking situations. The truck’s steering system utilizes a recirculating ball mechanism, offering precise and responsive steering.
Market Reception & Comparison
The 2015-2017 GMC Sierra 2500HD with the 6.6L Duramax and Allison automatic was well-received by critics and customers alike. It was praised for its exceptional towing capacity, robust powertrain, and comfortable ride. Fuel economy, while not a primary concern for heavy-duty truck buyers, was considered acceptable for a vehicle of this size and capability. Compared to the gasoline-powered Sierra 2500HD models, the diesel offered significantly better fuel economy when towing heavy loads. Compared to the Chevrolet Silverado 2500HD, which shared the same platform and powertrain, the Sierra offered a slightly more upscale interior and exterior styling. Reliability was generally good, although some owners reported issues with the diesel particulate filter (DPF) system.
Legacy
The Duramax LML engine and Allison transmission pairing in the Sierra 2500HD has proven to be a durable and reliable combination. These trucks continue to be popular on the used car market, commanding relatively high prices due to their proven capabilities. The LML Duramax is known for its longevity and relatively low maintenance costs, making it a desirable engine for buyers seeking a long-lasting work truck. While newer heavy-duty trucks offer more advanced technology and features, the 2015-2017 Sierra 2500HD remains a capable and dependable option for those needing a powerful and reliable truck for towing and hauling.
