I have a 2008 LT 4.2L 4x4 3.42s, bone stock. I bought it with 36K on it and am about to hit 47K in 6 months of ownership. I always log/calculate my mileage and have found that the mpg as calculated on the DIC always reads anywhere from 0.2-1.0mpg high.
When I got it back in the fall, I could usually see about 17-18 on a good day and about 14-16mpg over the winter when the cold weather hit. Since the weather has been getting nicer and I don't need to warm up or use the defroster, its already back on the upswing again for spring.
I averaged 13.X mpg on a tank like twice, but usually the lowest is 14-15mpg. The best I ever got was 23.4mpg on the way to the beach last fall according to the DIC, but I ended up cruising around in traffic in Ocean City, MD for cruise week on the rest of that tank, so the final numbers were low. But my average on the DIC was pretty close to my calculated average, so I was happy with that. I was keeping it at 65mph on cruise that whole time, and it was still going up before we got into town.
The best I got without trying to keep a particularly light foot was 21mpg on the way to Long Island over Christmas to visit family. 75-85mph the whole way, no cruise control. That wasn't too shabby. Seems like 65mph is the sweet spot for me.
Overall, I'm pretty happy with it. I really think the driver has a lot to do with it. I've always seemed to get pretty satisfying mpg in all of my cars. People need to realize there is more to getting good mpg than just keeping at a particular speed, or rpms. Every little nuance of your foot pressure on the gas pedal will drastically impact your mpg. If you have access to a OBD2 scanner, hook it up and monitor your throttle position or the engine load, and you will see how it affects your mileage. That's what I've done, and now I subconsciously am aware of how hard to push and when.