The GMC Sierra 3500HD IV (GMTK2XX) Double Cab Long Box, equipped with the 6.6-liter Duramax Turbo Diesel V8 engine producing 397 horsepower and paired with a 4WD SRW Automatic transmission, was a key offering in GMC’s heavy-duty truck lineup from 2015 to 2017. This configuration represented a balance between capability and usability, targeting customers needing substantial towing and hauling capacity without necessarily opting for the highest trim levels. It sat within the fourth generation of the Sierra 3500HD, known as the GMTK2XX platform, which brought significant improvements in interior refinement, technology, and overall robustness compared to its predecessors.
Technical Specifications
| Brand | GMC |
| Model | Sierra 3500HD |
| Generation | Sierra 3500HD IV (GMTK2XX) Double Cab Long Box |
| Type (Engine) | 6.6 Duramax TD V8 (397 Hp) 4WD SRW 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 | 8.5 kg/Hp, 117.8 Hp/tonne |
| Weight-to-torque ratio | 3.3 kg/Nm, 307.6 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 | 3371 kg (7431.78 lbs.) |
| Max. weight | 5262 kg (11600.72 lbs.) |
| Max load | 1891 kg (4168.94 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 | 1980 mm (77.95 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) | 223 mm (8.78 in.) |
| Minimum turning circle (turning diameter) | 16.2 m (53.15 ft.) |
| Drivetrain Architecture | All wheel drive (4×4). ICE drives rear wheels, front wheels engaged via clutch. |
| Number of gears and type of gearbox | 6 gears, automatic transmission Allison |
Powertrain & Engine Architecture
The heart of this Sierra 3500HD variant is the Duramax LML 6.6-liter turbocharged diesel V8. This engine, a staple of GMC’s heavy-duty trucks, is renowned for its durability and torque output. The LML iteration introduced several refinements over previous Duramax engines, including a redesigned combustion chamber, improved fuel injectors, and a more efficient turbocharger. The engine utilizes a common-rail direct fuel injection system, delivering fuel at extremely high pressures for optimized combustion. The overhead valve (OHV) valvetrain, while seemingly dated compared to overhead cam designs, proved robust and reliable in this application. The Allison 1000 six-speed automatic transmission is specifically designed to handle the immense torque produced by the Duramax engine. It features a lock-up torque converter for improved fuel efficiency and a heavy-duty cooling system to prevent overheating under demanding conditions.
Driving Characteristics
The 397 horsepower and 765 lb-ft of torque delivered by this combination provided substantial pulling power. While not as quick accelerating as gasoline-powered trucks, the Duramax offered a broad, flat torque curve, making it exceptionally capable for towing and hauling heavy loads. The Allison transmission’s gear ratios were optimized for both work and highway cruising. Lower gears provided ample torque for starting heavy loads or climbing steep grades, while higher gears allowed for relaxed and efficient highway travel. Compared to the 6.0L gasoline V8 option, the Duramax offered significantly more torque and better fuel economy, especially when towing. However, it lacked the immediate responsiveness of the gasoline engine. Compared to higher output Duramax variants, this 397hp version offered a more manageable power delivery, appealing to drivers who prioritized usability over outright performance.
Equipment & Trim Levels
The Double Cab Long Box configuration offered a spacious interior with seating for five passengers. Standard features typically included air conditioning, power windows and locks, a basic audio system, and vinyl upholstery. The SLT trim, a common pairing with this powertrain, added features like leather upholstery, heated front seats, a premium audio system, and a navigation system. Optional extras included a sunroof, rear-seat entertainment system, and various towing packages. The Double Cab configuration provided a useful rear seat area for passengers, while the Long Box offered ample cargo space for work or recreational needs.
Chassis & Braking
The Sierra 3500HD featured a fully boxed frame designed for heavy-duty use. The front suspension utilized coil springs and a double wishbone design, 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 even when towing heavy loads. An Anti-lock Braking System (ABS) was standard. The rear axle was a single rear wheel (SRW) configuration, suitable for most heavy-duty applications.
Market Reception & Comparison
The GMC Sierra 3500HD with the 6.6L Duramax and Allison automatic was generally well-received by critics and consumers alike. It was praised for its robust construction, powerful engine, and comfortable ride. Fuel economy, while not exceptional, was competitive for a heavy-duty truck. Compared to the Ford Super Duty and Ram 3500, the Sierra 3500HD offered a similar level of capability and refinement. Some reviewers noted the GMC’s interior was slightly more upscale than its competitors. Reliability was generally good, although diesel engines can be more expensive to maintain than gasoline engines due to the complexity of the fuel injection system and emissions controls.
Legacy
The GMTK2XX generation Sierra 3500HD, and specifically this 6.6L Duramax/Allison combination, has proven to be a durable and reliable truck. Used examples continue to command relatively high prices due to their reputation for longevity and capability. The LML Duramax engine is considered one of the most reliable Duramax engines produced, and the Allison transmission is known for its durability. Common maintenance items include regular oil changes, fuel filter replacements, and occasional repairs to the emissions system. These trucks remain popular choices for those needing a capable and dependable heavy-duty work truck, and well-maintained examples can provide many years of service.
