I have a 96 Super cab 350 dually 5 speed. I getright at 15.7 on almost every tank. Im looking at a 99 late 99. Crew cab dually with a 7.3 with an automatic. Im wondering how much milage differance there will be in the 2 trucks. The 99 is a 4X4 and the 96 is a 2X4.