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 an Allison automatic transmission, represents a significant iteration in GMC’s heavy-duty truck lineage. Produced from 2015 to 2017, this variant occupied a sweet spot in the Sierra 3500HD lineup, offering a compelling blend of towing capacity, payload capability, and everyday usability. It was positioned as a workhorse for those needing serious capability without necessarily opting for the highest trim levels or specialized configurations. This generation Sierra 3500HD, built on the GMTK2XX platform, aimed to improve upon its predecessor with enhanced refinement, technology, and overall performance.
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) 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 | 8.2 kg/Hp, 121.7 Hp/tonne |
| Weight-to-torque ratio | 3.1 kg/Nm, 318 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 | 3261 kg (7189.27 lbs.) |
| Max. weight | 5080 kg (11199.48 lbs.) |
| Max load | 1819 kg (4010.21 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 | 1981 mm (77.99 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) | 221 mm (8.7 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, 373×34 mm |
| Assisting systems | ABS (Anti-lock braking system) |
| Steering type | Cone worm with recirculation balls |
| Tires size | 235/80 R17; 265/70 R18 |
| Wheel rims size | 7.5J x 17; 8J x 18 |
Powertrain & Engine Architecture
The heart of this Sierra 3500HD is the Duramax LML 6.6-liter turbocharged diesel V8. This engine, a cornerstone of GMC’s heavy-duty offerings, is renowned for its durability and substantial torque output. The LML iteration of the Duramax introduced several refinements over its predecessor, including a redesigned turbocharger, improved fuel injection system, and enhanced cooling. The engine utilizes a common-rail direct injection system, delivering fuel at extremely high pressures for optimized combustion. The overhead valve (OHV) valvetrain, while traditional, is robust and contributes to the engine’s longevity. The Allison 1000 six-speed automatic transmission is specifically designed to handle the immense torque produced by 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 demanding conditions. The transmission’s gear ratios are optimized for both towing and hauling, providing a balance between power and efficiency.
Driving Characteristics
The 397 horsepower and 764.85 lb-ft of torque delivered by this combination translate into impressive real-world performance. While not as overtly quick as some gasoline-powered trucks, the Duramax/Allison pairing provides effortless acceleration, even when heavily loaded. The Allison transmission shifts smoothly and decisively, making it a comfortable daily driver. Compared to the 6.0L gasoline V8 option, the Duramax offers significantly more low-end torque, making towing and hauling much easier. Compared to higher-output Duramax variants (typically found in higher trim levels), this 397 hp version offers a slightly more relaxed driving experience, prioritizing fuel economy and longevity over outright performance. The gear ratios in the Allison transmission are well-suited for maintaining highway speeds while towing, and the engine remains relatively quiet and refined even under heavy load.
Equipment & Trim Levels
The 6.6 Duramax with the Allison automatic was available on various trim levels of the Sierra 3500HD, including SLE and SLT. Standard features typically included air conditioning, power windows and locks, a basic audio system, and vinyl 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, rear-seat entertainment, and various towing packages. The Double Cab configuration provided ample space for passengers, making it a practical choice for both work and family use. The long box offered a generous cargo bed for hauling materials or equipment.
Chassis & Braking
The Sierra 3500HD’s chassis is built for 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. The braking system features ventilated discs at all four corners, providing ample stopping power even when towing. An Anti-lock Braking System (ABS) is standard, enhancing safety and control. The rear axle ratio varied depending on the specific configuration, but typically ranged from 3.73 to 4.10, influencing both towing capacity and fuel economy.
Market Reception & Comparison
The GMC Sierra 3500HD with the 6.6 Duramax and Allison automatic received generally positive reviews from automotive critics. It was praised for its powerful engine, smooth transmission, and comfortable ride. Fuel economy, while not exceptional, was considered reasonable for a heavy-duty truck. Compared to the Ford Super Duty and Ram 3500, the Sierra 3500HD was often seen as offering a more refined interior and a quieter cabin. The Duramax engine’s reputation for reliability also contributed to its appeal. However, some critics noted that the Sierra 3500HD’s price was slightly higher than its competitors.
Legacy
The Duramax LML engine paired with the Allison transmission has proven to be a remarkably durable and reliable powertrain. Properly maintained, these engines can easily exceed 200,000 miles. On the used car market, Sierra 3500HDs equipped with this powertrain command a premium price due to their reputation for longevity and capability. While newer heavy-duty trucks offer even more power and technology, the 2015-2017 Sierra 3500HD remains a popular choice for those seeking a capable and dependable work truck. The combination of a proven diesel engine, a robust transmission, and a comfortable cabin ensures that this variant will continue to be a valuable asset for years to come.
