My '01 2500HD RCLB with the LQ4/4L80/4x4/4.10/245x75x16 bone stock gets about 12-13mpg city and about 14-15mpg highway. I drive the truck harder now than I used to since I drive far less highway than I did back when I first got the truck, but back in the day I'd average 17+mpg driving like an old woman on the freeway. The best I've ever gotten was 21mpg coming back on I-10 during a freaking crazy wind storm, which is exactly what I attribute the mileage increase from - 40 to 50mph winds pushing you really helps out a lot .

Tom