The Allison 1000 has been the standard automatic transmission for the Duramax diesel since its inception in 2001. At this time, transmission's size, strength, and versatility far exceeded anything available from the Duramax's direct competitors. The Allison 1000 was a five speed transmission from 2001 to 2005 model years, while a significant upgrade for the 2006 MY included the addition of a second overdrive gear, making it a six speed transmission. Allison's specialty is commercial grade automatic transmissions for medium and heavy duty applications, which includes everything from transit buses to tractor trailers. Acquired by General Motors in 1929 following the death of the company's founder, Allison's history includes the development of propulsion systems for aircraft and other military applications prior to the end of the WWII. GM sold Allison Transmission in 2007, although Duramax powered pickups continue to use their transmissions.

Moderate changes have occurred to the transmission throughout the years however the platform itself has, for all intents and purposes, remained the same since 2001. The most notable modification has been the transition from a five to six speed model, which lowered cruising rpm by roughly 14% in high gear. This addition is particular attractive in terms of fuel consumption, as the lower engine speeds create an opportunity for greatly improved fuel economy on the highway. Rather than becoming outdated, the Allison has evolved to match the performance characteristics and demands of the Duramax turbodiesel, which has undergone significant changes itself through the years.

Manual transmissions are relatively rare for Duramax powered GMC/Chevrolet pickups, the primary reason being the attractiveness of the Allison 1000. General Motors was the first of the "Big 3" to phase out a manual transmission option for their diesel - the ZF 6 speed was last offered for the 2006 model year. Modern automatic transmissions, such as the Allison 1000, are far superior to manual transmissions in almost every category. While a manual transmission is obviously a much simpler machine, there can be no integration between engine parameters and transmission shift schedule; such communication between powertrain components brings us innovative features that include engine braking and dynamic shift schedules. Additionally, the Allison 1000 is rated with a far greater input torque capacity than any outgoing manual transmission.

The Allison's external filter eliminates the need to frequently service the internal filter and greatly reduces maintenance time/cost. It also relies on adaptive technology which integrates the transmission with the engine control module to continuously monitor driver input and calculate ideal shift strategies for the particular driving conditions.