I used to have a Silverado SS which was the same truck except extended cab. I was getting about 16mpg, with exhaust and an intake on it. Ran the original tires about 43,000 miles, only replaced them because I got puncture on one of them.
You should like the 6.0 in this truck better than the hummer, the Hummer would have the LQ4 motor in it. The Denali will have the LQ9 which is a higher compression version of the LQ4 (10:1) Horsepower and torque are higher in the LQ9 as well.