The smallest trucks the 3.9 came in was over 4000 lbs. That little a body will knock 1000 lbs off that plus be more aerodynamic. There are a few things you could do to improvwe the mpg over and above that. The injectors have terible targeting and spray pattern, some accel injectors with there cone type spray pattern should help as well as upping the compression and quench. There are a couple ways to do that, you can put in a set of KB 167 pistons and they should be at zero deck with a light deck surfaceing or you can just mill the deck of the block .070 and use the stock flat top pistons, throw in new rings and bearings and your set. Those couple things will help efficency and power a bunch.