As another data point in this thread...2013 Silverado 2500HD, 4x4, Duramax, ECSB, stock 20" wheels and tires, Lance 815, GVW about 10K lbs loaded.
In a mix of interstate and back roads I typically see about 15.5-16.5mpg. On back roads, I'm usually running 50-60 and on limited access, 65-75...