I can get 20-22mpg easy if it's all highway. I have 33" mud tires and 20" wheels (heavy) so it's not as good as it could be. Before I got these wheels/tires I had some all-terrain tires on 17's wheels and one time when I was reeeally babying it on the highway I got 26mpg.
With that being said, my fuel is cheap now so I have paid much less attention to how I drive. I usually get 16-18mpg mixed and I'm not careful with the right foot. I have a 5" turbo back exhaust and I like to listen to it
Truck is a 2007.5 Chevy 2500HD Duramax 4x4. Tuned with exhaust but thats pretty much it for engine mods.