I get 14/18 in my Yukon XL at about 6ooo lbs. . I installed the ram air, exhaust, and, power programer III and found that the Programmer III seemes to make the best incease in economy. I wish I got the 4.10 rear gear in stead of the 3.73. due to the weight and size of the tires . I plan on installinig 20 Inch rims and maybe the Keen Bell SuperCharger to see how the towing will do. I need to be doing 72> mph to get decent power on my 5000lb car trailer (kids carts 20x 8.5). I would likr to get some feedbak of what I can expect. Should I go to 4.10 gears .