* Allison 1000 6-speed automatic transmission: This was the standard transmission for the Silverado 2500HD and was known for its smooth shifting and heavy-duty capabilities.
* 6-speed manual transmission: This was an optional transmission that offered more control and fuel efficiency for those who preferred it.
It's important to note that the specific transmission in your Silverado 2500HD depends on the trim level and engine you have. You can find this information on your vehicle's identification number (VIN) sticker or by contacting a Chevrolet dealership.