The transmission depends on the engine and configuration. Gasoline models with the 6.0L Vortec V8 typically used a 4-speed automatic (4L80-E). Diesel models with the 6.6L Duramax LB7 used a 5-speed automatic (Allison 1000). A rare manual option (NV4500 5-speed) was available on some heavy-duty trims. Exact configuration varies by cab/bed and option codes.
In 2004, the Sierra 2500 lineup offered both gasoline and diesel powertrains, and the transmission pairing was closely tied to the chosen powertrain. The two most common automatic transmissions are well documented for this model year, with a manual option appearing on select configurations. To know the exact setup of a specific truck, check the VIN or build sheet.
Transmission options by powertrain
The following list summarizes the typical transmissions paired with each powertrain for the 2004 Sierra 2500 series.
- Gasoline 6.0L Vortec V8: 4-speed automatic transmission 4L80-E.
- Duramax diesel 6.6L LB7: Allison 1000 5-speed automatic.
- Manual option: NV4500 5-speed manual available on some heavy-duty configurations; not universal—verify with the VIN or build sheet.
Notes: Availability varied by cab configuration, bed length, and market. Duramax-equipped trucks are typically found in the Sierra 2500HD/2500HD lineage, while gasoline-powered models more commonly used the 4L80-E automatic. Always confirm the exact transmission of a given vehicle via its VIN or original documentation.
Manual transmission specifics
The NV4500 is GM’s heavy-duty 5-speed manual transmission used in certain 2004-era 2500HD configurations. If your truck has a manual gearbox, it is likely the NV4500 and should be verified against the vehicle’s option codes. VIN and build data remain the best sources for confirmation.
Summary
In brief, a 2004 GMC Sierra 2500 could come with either a 4-speed automatic (4L80-E) on most gasoline 6.0L Vortec engines, a 5-speed automatic (Allison 1000) on Duramax diesel versions, or a rare 5-speed manual (NV4500) on select heavy-duty configurations. To determine the exact transmission on a specific truck, consult the VIN or original documentation.