Been my experience most guys pad their real MPG 3-5 mpg to justify the expense of owning a miniture big rig. An 06 is a bad one for MPG as it is a very heavy truck with a 24 valve that never seemed to do as good as the 12V it replaced anyhow. Some chips can help significantly, otherwise, Check your boost, and fuel system out if you don't find anything than enjoy the truth.

I am not causing global warming, I am just trying to hold off a impending Ice Age!