The only reason you could possibly be getting poor milage was if you had the 4.10 gears vs. the 3.73's. I have an identical truck with the smae mods and tire size. With the winter fuels im still getting about 15. During the summer i was getting around 18 in the city and around 21 on the highway. All of these readings were not towing anything. I bought my truck a year ago with 38000 miles on it. So i would have to say that your engine is broke in, as mine was at 38000. Check the tag on yur diff and that will tell you your gear ratio. Thats about all I can come up with. Good luck.