I also have the 265's on my truck, I have a stock rear. The last I checked at fill up was 12.7 for an average. But I am about 35% highway 65% town driving. I think a 3.9 with running 225's & 2wd drive may hit 17-18, but these V-8 4X4's are generally hard on fuel. When gas went throuh the roof a year ago I tried
feathering the throttle... slow take off's, nothing worked. same mpg's