I've had mine for 2 years now and never had a problem with mileage - 15/18. I use it a lot in the woods and never had a problem on or off road. I do agree the tires are too small for the truck (that was the very first thought when I saw it). But it's been a good truck that handled well on a WA to NJ road trip of 4000+ miles with a motorcycle and stuff in the bed ranging from twisting turning roads in Yellowstone to 90+ MPH straightaways in SD (milage goes to 14-15 at 90MPH).
This is the first non big block gas truck that will actually tow and haul large loads without having a gear too high and a gear too low to tow with. The 6 speed with 4:10 gears works great and doesn't hunt. I can haul my slide in camper with a towed or hook on to the 30ft enclosed 6 place sled trailer and head to the mountains 1200 miles away. It handles remarkably well for its hulk size and the ride quality is excellent. I get 12-13 in the city and 14-16 on the highway which I can't complain. I had a 1500 5.3 which did great but I couldn't do anything with it. I have had absolutely no issues with this truck and love it.
Great truck if you want to turn people's head. Mileage not as good as the 2005 I traded in. Both were equiped the same way. Maybe it will get better. My 05 keep improving till about 35,000 miles. Interior much improved. The navagation is really nice to have, I also have it on my 07 1500. My crew that works out of it really do love driving the 08 LTZ dully. It does every thing we ask and more.
This truck I bought last year new. I have already 65,000 miles on it. It broke at about 56,000 and now I'm getting 18MPG without the trailer on the highway. At first it got 12 and 14 at best with out the trailer. I haul 10,000-13,000 pounds all the time. You can go up and down moutains and it just keeps on moving forward. The Allison Trany and the Duramax makes the truck tougher and more reliable. It never broke down on me/never had a problem. I am always going to be a Chevy fan. The tow command is simple and very good to have. It helps out buy being easier on my truck brakes. Always has power to spare. I would recommend this truck to anyone that needs a tough reliable truck that will never stop.