These are the only transmissions available for the 6.6L engine. The manual is a 5 speed with a granny gear.
ZF S6-650 6-Speed Manual Gear Ratios
5.79:1 Low
3.31:1 First
2.10:1 Second
1.31:1 Third
1.00:1 Fourth
0.72:1 Overdrive
Allison 1000 Automatic Gear Ratios
3.10:1 First
1.81:1 Second
1.41:1 Third
1.00:1 Fourth
0.71:1 Overdrive
0.61:1 2nd Overdrive (2006+ models)
A 6 speed conversion is available for earlier model 1000's. It does not require any other modifications. $1648.00 +$500 core charge.
The cost of a stock 6 speed is compareable but I don't know if any other parts are needed to go that route.
http://www.suncoastconverters.com/gmc/gm_products.html
All 2500HD with 6.6L have an AAM 11.5” axel with 3.73 ring & pinion and Eaton G80 locking differential.
Available ratios
3.42, 3.73, 4.11, 4.56, 4.63, 4.88, 5.13
Ring and pinion sets start at $350
Shim and seal kits start at $80
Master kit w/ bearings start at $200
Stock tire size is 245/75R16 (30.5” dia.)
As previously stated 3.42 gear change will take place in near future and the 6 speed conversion or trans swap is a distant goal.