My brother's boss just bought a 2001 HD crew cab 2500 4x4 short bed.It has the 6.0 auto and he's getting about 12 mpg,it only has 1600 miles on it now.He wishes he got the 8100/Allison since it is only 1 or 2 mpg worse.
Got a 2500HD with 6.0L and 12mpg is about right. Havn't been on too many long trips yet to see if it improves much over the long haul. Pulling a trailer (7x14, about 7,000lbs worth of tools) it goes down, probably in the 8-9 range, but never really could compute that out yet, as it may be even lower.
As for the 8.1 liter, i've heard this. If you go with the 8.1 and 3.73 gear, guys are saying they are averaging almost 12 mpg also. So, compared to my 6.0L with 4.10's, its amost Exactly the same (heard of a .3 mpg difference) Some Food for thought! Also, with the power in the 8.1, it has no problem turning those 3.73's, so performance is about the same at the low end as the 6.0L, but you have that extra humph up top for hauling.
Kind of makes me wish I heard this earlier as I may have opted for the 8.1 instead, as I commute 70+ miles a day to and from full time job.