Originally Posted by
Kennedy
OK guys stop and take a deep breath here. Dennis has a Medium duty vehicle built as a 6 speed but calibrated as a 5 speed. The pickups are either 5 or 6 speeds. The 01-05 can be converted to 6 speed by using a custom programmed TCM (proprietary software using unreleased stolen? calibrations from Allison) and a modified valve body.
I did this to my 05 years back. I don't really think it gained me much in terms of MPG, but it does make things quieter. After driving a 6 speed truck and then switching to a 5 speed it just seems "busy" with too much engine speed at cruise.
P.S. the pre-Bosch ECM trucks do not hold 6th as well as the post. Be it superior TQ or simply programming differences, the LBZ up series holds 6th much better and responds much better to increasing loads.