I don't have specific experiance with an LS3 however I have had an LS1 C5, an LS2 C6 with and without a cam and now an LS7 Z06 and they all got/get about the same mileage. All of them easily got over 30mpg on the highway but you aren't asking about that. Around town I normally average 15-18 mpg with strictly city and or stop and go traffic and 20-24 if I mix in some highway. Now if I have any spirited driving days that number drops to 10-12 mpg with around 7-8 mpg on the track. I have found that you never really need to go over 2k rpms to keep up with and honestly still outpace traffic. Keeping it under 2k as I putt around town keeps that mpg up pretty high but even with normal driving like you are mentioning I am definetly closer to 20mpg than farther. Again, this has been my experiance in all of my Vettes give or take 1 or 2 mpg's. Give the car some time to break in and the ECU to learn and adjust.