I feel like that average is really low. Talking with my buddy who is a trucker, he said that 6mpg is on the low end. His truck, which isn't anything special, would usually get 8-9. He said those guys getting 6 or lower are usually owner / operators who have older trucks, either because they like the older trucks or they can't afford to get a new one. Most fleets have much newer trucks, a lot of which can get up to 10mpg or more. That may not seem like a huge improvement, but over thousands of miles it adds up really fast.