2007-2010 GMC Sierra 2500HD III (GMT900) Crew Cab Long Box 6.6 Duramax TD V8 (365 Hp) Automatic

The GMC Sierra 2500HD III, specifically the Crew Cab Long Box configuration powered by the 6.6-liter Duramax turbodiesel V8 engine paired with an Allison automatic transmission, represents a significant iteration in GMC’s heavy-duty truck lineage. Produced from 2007 through 2010 (GMT900 platform), this variant was aimed at buyers needing substantial towing and hauling capacity without sacrificing interior space and comfort. It occupied a sweet spot in the 2500HD lineup, offering a balance between the more economical gasoline engines and the higher-output Duramax variants.

Technical Specifications

INTRODUCTION

The 2007-2010 GMC Sierra 2500HD Crew Cab with the 6.6L Duramax diesel and Allison automatic transmission was a cornerstone of GMC’s heavy-duty truck offerings. Built on the GMT900 platform, this generation represented a significant step forward in terms of interior refinement, towing capacity, and overall capability. The Crew Cab configuration provided ample space for passengers, making it suitable for both work and family use. This particular variant, with its 365 horsepower and 660 lb-ft of torque, was positioned as a strong contender for those needing serious power but perhaps not requiring the ultimate output of the higher-tuned Duramax engines.

Powertrain & Engine Architecture

At the heart of this Sierra 2500HD lies the Duramax LMM engine, a 6.6-liter turbocharged diesel V8. The LMM represented an evolution of the previous LBZ engine, incorporating a more efficient combustion system and improved emissions controls. It featured a cast iron block and aluminum cylinder heads, along with a robust crankshaft and connecting rods designed to withstand the stresses of heavy-duty operation. The engine utilized a high-pressure common rail fuel injection system, delivering fuel directly into the cylinders at extremely high pressures for optimal combustion. The valvetrain employed an overhead valve (OHV) configuration, known for its simplicity and durability. The Duramax LMM was paired exclusively with the Allison 1000 series 6-speed automatic transmission. This transmission, renowned for its strength and reliability, was specifically designed to handle the high torque output of the Duramax engine. It featured a lock-up torque converter for improved fuel efficiency at highway speeds and a robust internal design to ensure long-term durability.

Driving Characteristics

The 365 hp/660 lb-ft combination in the Sierra 2500HD delivered a compelling driving experience. While not as overtly powerful as the higher-output Duramax variants, this setup provided ample torque for towing and hauling heavy loads. The Allison transmission shifted smoothly and predictably, making it easy to manage power delivery. The gear ratios were optimized for both low-end grunt and comfortable highway cruising. Compared to gasoline-powered Sierra 2500HDs, the diesel offered significantly more low-end torque, making it easier to accelerate with a heavy load. Compared to the higher horsepower Duramax versions, this variant felt more manageable and less prone to wheelspin, especially in slippery conditions. The overall driving experience was characterized by a sense of solidity and capability, befitting a heavy-duty truck.

Equipment & Trim Levels

The Crew Cab Long Box 2500HD typically came standard with features like air conditioning, power windows and locks, and a basic audio system. Higher trim levels, such as the SLT and Denali, added features like leather upholstery, heated seats, a premium audio system, and chrome exterior trim. Optional extras included a navigation system, a sunroof, and various towing packages. The interior of the Crew Cab was spacious and comfortable, with ample legroom for both front and rear passengers. The dashboard layout was functional and straightforward, with easy-to-read gauges and controls.

Chassis & Braking

The Sierra 2500HD featured a robust chassis designed to withstand the stresses of heavy-duty use. The front suspension utilized an independent torsion bar setup with double wishbones, providing a comfortable ride and good handling. The rear suspension employed a traditional leaf spring design, offering excellent load-carrying capacity. Braking duties were handled by ventilated disc brakes on all four wheels. Anti-lock braking system (ABS) was standard. The rear brakes were slightly smaller in diameter than the front brakes, reflecting the weight distribution of the truck. The steering system utilized a recirculating ball mechanism, providing a solid and predictable feel.

Market Reception & Comparison

The GMC Sierra 2500HD with the 6.6L Duramax and Allison automatic was generally well-received by critics and consumers alike. It was praised for its powerful engine, smooth transmission, and comfortable interior. Fuel economy, while not exceptional, was respectable for a heavy-duty truck. Compared to the gasoline-powered Sierra 2500HDs, the diesel offered significantly better fuel economy when towing or hauling heavy loads. Compared to the higher-output Duramax variants, this version offered a more affordable price point and a slightly more refined driving experience. Reliability was generally good, although some owners reported issues with the emissions control system.

Legacy

The 2007-2010 GMC Sierra 2500HD with the Duramax LMM engine and Allison transmission remains a popular choice in the used truck market. The Duramax LMM engine is known for its durability and longevity, and the Allison transmission is widely regarded as one of the most reliable automatic transmissions ever made. These trucks are often sought after by those needing a capable and reliable work truck or a comfortable and spacious family vehicle. Properly maintained examples continue to command respectable prices, reflecting their enduring reputation for quality and capability. The LMM Duramax, while superseded by later iterations, remains a highly regarded engine among diesel enthusiasts.