If you are doing true city driving, you probably won't get too much better than that. I don't think you would be having winter blend fuel down there in TX, so you can't blame that. You will find out that a good majority of people don't hand calculate their fuel mileage, they just go by the DIC. This becomes a problem when you start adding a programmer, which throws the DIC off even more. If I would go by my DIC, I would be getting about 24-28. In actuallity I usually get 17-19. During the winter up here I get anywhere from 10-14 depending how much I drive into town (21 miles one way). Do you have a dually? As far as what you are supposed to get....I don't know for sure, but my LB7, I get anywhere from 17-19 on with mostly highway driving. LB7s did get better mileage than the LLY, but the LBZ hasn't been out long enough to tell. Of course a diesel engine isn't really considered "broke in" until around 25,000-30,000 miles.