Have a 2010 GMC 2500HD 4x4 crewcab with the 6.0 gasser and 3.73 gears. In the summer I can avg 14-15, winter it drops to 11. That is for combined city and country roads. I actually do better on 2 lane highways at 55-60 mph than on the interstate at 70 mph, can average 17-18 at 55-60 mph. Towing my 8000 lb travel trailer I avg'd 8 mpg towing from Ohio to Florida. Kentucky and Tennessee are fairly hilly and it was at 65-70 mph, tranny didn't do alot of shifting, don't remember the temps, so it must not of been bad enough for me to remember.
Seeing as how I'm a weekender and 1 big trip every year or so, the diesel didn't make sense, especially with fuel prices right now, gas around here is staying about $3.20, diesel is staying about $4.20.