Just traded my 04 2500HD LLY 38000 miles on a new 06 3500 LBZ Dually, drove from Denver to KC to drop off old truck and pick up new. 80 mph the whole way 570 miles.

Outbound trip in LLY 16.9 mpg overall
Return trip in LBZ 11-12 mpg for the first 300 miles, 15.3 mpg in the last 270.

Next day 100 miles of local driving 600 miles on odometer 55-65 mph, 15.9.

LLY 5 speed allison 80 mph, 2400 rpm
LBZ 6 speed allison 80 mph, 2050 rpm

Looking good to beat 04 mpg once fully broken in.