The Sun Coast A1-6VB-TCM-LB7 Allison 5-6 Speed Conversion for your 2001-2004 GM 6.6L Duramax LB7 converts your 5-Speed Allison A1000 Transmission into a 6-Speed like the later models. With the conversion, you can drop 325 engine rpm @ 75 mph with 3.73 gears. While increasing fuel economy 1-1.5 MPG, reduces engine noise while cruising, a truck with 3.73 gears that normally like cruising at 62 MPH will now cruise nicely at 72 MPH. The gear ratio in your 5 speed is .71 and our 6th gear will be .61. 6th gear can be disabled with your two haul button and 5th canceled with OD off. Requires valve body and TCM change. No wiring, no modifications required.