I have a 2008 Chevy 2500hd crew cab swb z71 and it has 6,500 mile on it I was getting about 12.5 to 13 some what city milage. I never get to check the hwy milage because i am always towing my boat on the highway. I just put a Hypertech programer on it 2 weeks ago and have only run 1 tank of gas thru it since then and it gave me 1.5 mpg better. I am filling up when I leave work today and will post what the last tank got.