I've got a standard cab with a 5.9 and auto and have nudged the mileage (and power to boot) from high 12's (with the FIPK) up to mid 16's mixed highway and city, and I don't baby it at all. The biggest gainers so far have been gibson headers/ flowmaster exhaust (I installed them within a day of each other so I don't know which was the big gainer) which was about 2 MPG, hypertech power programmer gained me about 1 MPG, and the MSD 6A gained me another .5 MPG. I did, however, swap all the trans fluid (with the truck running in N method and adding fluid at the rate it gets pumped out) with valvoline synthetic atf+4, and swap the engine oil to durablend semi synthetic when I did the headers/exhaust, so I think some of that gain was from that stuff too. I just did a modified kegger/hughes throttle body install, and will post new MPG readings, although I suspect they will be a bit low because I've been hot rodding it around a bunch.