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 an Allison automatic transmission, represents a significant iteration in GMC’s heavy-duty truck lineage. Produced from 2015 to 2017, this configuration occupied a sweet spot in the Sierra 2500HD lineup, offering a balance of robust towing capacity, comfortable interior space, and refined diesel performance. It was positioned as a workhorse for owners needing substantial capability without necessarily opting for the highest trim levels or the most extreme performance options.
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) 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 | 8.1 kg/Hp, 124 Hp/tonne |
| Weight-to-torque ratio | 3.1 kg/Nm, 323.9 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 | 3202 kg (7059.2 lbs.) |
| Max. weight | 4536 kg (10000.17 lbs.) |
| Max load | 1334 kg (2940.97 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 | 6321 mm (248.86 in.) |
| Width including mirrors | 2045 mm (80.51 in.) |
| Height | 1978 mm (77.87 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) | 216 mm (8.5 in.) |
| Minimum turning circle (turning diameter) | 16.2 m (53.15 ft.) |
| Drivetrain Architecture | 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 Double Cab Long Box with the 6.6L Duramax diesel and Allison automatic transmission was a key component of the GMTK2XX platform, representing the fourth generation of GMC’s heavy-duty truck series. This generation marked a significant step forward in terms of refinement, capability, and technology. The Double Cab configuration offered increased rear-seat space compared to the standard cab, making it suitable for both work and family use, while the Long Box provided ample cargo capacity. The Duramax/Allison pairing was a popular choice, known for its reliability and ability to handle demanding tasks like towing large trailers and hauling heavy loads.
Powertrain & Engine Architecture
At the heart of this Sierra 2500HD lies the Duramax LML 6.6-liter turbocharged diesel V8. This engine, a cornerstone of GMC’s heavy-duty lineup, utilizes a cast iron block and aluminum cylinder heads. The LML generation of the Duramax introduced a revised turbocharger, improved fuel injection system, and enhanced glow plug system for improved cold-start performance and reduced emissions. The engine employs a diesel commonrail fuel injection system, delivering highly pressurized fuel directly into the cylinders for efficient combustion. The overhead valve (OHV) valvetrain, while traditional, proved robust and reliable. The Allison 1000 six-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 and a heavy-duty cooling system to prevent overheating under strenuous conditions. The transmission’s gear ratios are optimized for both towing and everyday driving, providing a balance of power and efficiency.
Driving Characteristics
The 397 horsepower and 765 lb-ft of torque delivered by the Duramax/Allison combination provided substantial performance. While not as overtly powerful as some gasoline-powered alternatives, the diesel engine’s torque curve offered exceptional pulling power, particularly at lower RPMs. The Allison transmission shifted smoothly and predictably, contributing to a comfortable driving experience even when towing heavy loads. The gear ratios were well-matched to the engine’s characteristics, allowing for effortless acceleration and maintaining speed on highway inclines. Compared to the 6.0L gasoline V8 option, the Duramax offered significantly more torque and better fuel economy, especially when towing. However, the gasoline engine was generally more responsive at higher RPMs. Compared to the 6.6L Duramax in higher trim levels (which could exceed 400 hp), this version prioritized usability and fuel efficiency over outright power.
Equipment & Trim Levels
The 2500HD Double Cab Long Box with the Duramax engine was typically offered in SLE and SLT trim levels. The SLE trim included standard features such as air conditioning, power windows and locks, a basic audio system, and cloth upholstery. The SLT trim added features like leather upholstery, heated front seats, a premium audio system, and additional chrome exterior trim. Optional extras included a navigation system, a rear-seat entertainment system, and various towing packages. The interior design was functional and durable, with a focus on providing a comfortable and practical environment for both work and family use.
Chassis & Braking
The Sierra 2500HD featured a robust chassis designed to withstand heavy loads. The front suspension utilized coil springs and a double wishbone configuration, while the rear suspension employed leaf springs. This setup provided a comfortable ride while maintaining excellent load-carrying capacity. The braking system consisted of ventilated discs on all four wheels, providing ample stopping power. An Anti-lock Braking System (ABS) was standard. The rear axle ratios varied depending on the specific configuration, but were generally geared for towing and hauling. The steering system utilized a cone worm with recirculation balls, offering precise and responsive steering control.
Market Reception & Comparison
The GMC Sierra 2500HD with the Duramax/Allison combination received generally positive reviews from critics. It was praised for its powerful engine, smooth transmission, comfortable ride, and impressive towing capacity. Fuel economy was considered good for a heavy-duty truck, especially when compared to gasoline-powered alternatives. However, some critics noted that the diesel engine was relatively noisy compared to some competitors. In terms of reliability, the Duramax/Allison pairing had a strong reputation, although some owners reported issues with the emissions control system. Compared to the Ford Super Duty and Ram 2500, the Sierra 2500HD was often seen as a more refined and comfortable option, while the Ford and Ram offered more aggressive styling and potentially higher towing capacities in certain configurations.
Legacy
The 2015-2017 GMC Sierra 2500HD with the 6.6L Duramax and Allison automatic transmission remains a popular choice in the used truck market. The Duramax/Allison powertrain is known for its longevity and reliability, making these trucks a good value for buyers seeking a capable and dependable workhorse. While maintenance costs for a diesel engine can be higher than for a gasoline engine, the Duramax engine is generally considered to be relatively easy to maintain. These trucks continue to be sought after by owners who need a truck that can handle demanding tasks without compromising on comfort or refinement. The LML Duramax engine, in particular, is regarded as one of the most reliable and durable diesel engines ever produced.
