The transmission in a 2008 GMC Sierra 2500HD depends on the engine: diesel versions use the Allison 1000 six-speed automatic, while gasoline versions use a GM four-speed automatic (4L80-E family); there was no factory manual transmission option for this model year.
Transmission options by engine
Diesel option
The diesel-powered Sierra 2500HD pairs with a rugged, torque-ready automatic designed for heavy work.
- Allison 1000 6-speed automatic transmission
In practical terms, the Duramax-powered 2008 Sierra 2500HD was equipped with the Allison 1000 six-speed automatic to handle high-torque towing and payload demands.
Gasoline option
The gasoline version uses a traditional heavy-duty four-speed automatic suited for everyday hauling and highway driving.
- GM 4L80-E four-speed automatic (4L80-E family)
For the 6.0L Vortec gasoline engine, the four-speed automatic provided reliable performance for lighter-duty workloads compared with the diesel setup.
Summary
Across the 2008 GMC Sierra 2500 lineup, transmission choice was engine‑dependent. The Duramax diesel used the Allison 1000 six-speed automatic, while the 6.0L Vortec gas engine used a four-speed automatic from the 4L80-E family. No manual transmission option was offered for this model year, and buyers could expect different driving dynamics and towing capabilities based on the selected powertrain.