I have a 2003 2500.....I get about 13.5 on the open road.
A little less when driving in the Rockies. I also carry a payload of about 900lbs in the bed. I am guessing you could do a little better with a lighter load. I am acually pretty happy with my 13mpg. I got 9 adn 11 at the most on my 2500 Suburban.