There isnt a lot that can be done about the poor mileage of the 6.4l. You should start by having the PCM updated with the latest calibration.Ford has come out with a new one that has shown a small increase in economy,it reduces the time that the engine will go into regen mode,which really burns up fuel.Also switching to a synthetic 0-w30 or 0-w40
oil will help.
Ford Master Tech
30+ years in trade,Ford Master Tech,Journeyman Automotive Tech