Pete,
I do have the LLY with the 6 speed. Just the last 2 days driving my short distance to work, no towing, I am getting right at 16 mpg. I sold my LMM 2007 partially to get better mileage because I became tired the the regen horrible mileage and the fact that I could not do any real substantive aftermarket mods without getting flashes of check engine messages.
Mark
2007.5 GMC SLE 2500 HD LMM **SOLD**
Owned 2006 GMC Sierra CC LLY/LBZ *2 not a D in SN* 214K Purchased 3/2010
CC/SB/4x4 Summit White
Owned a 2002 GMC CC LB7 CC sold with 135K
Now 2015 GMC Crew Cab Duramax/Allison
Custom Tool box