the main thing for mileage is similar to making power....more power, less throttle needed for mileage......make sure you have a CAI (cold air intake) and at least a full 4" mandrel bent exhaust.....I run K & N and Banks stuff.....I pull down around 24 mpg in my 2005 Cummins unloaded w/4.10 gears (2500)....go to the diesel forum sites and you can get unlimited info for darn near anything.....
www.cumminsforum.com
hey Art....there is a LOT you can do with your F550.....just do a general word search for "powerstroke forum" lots of good info there.....also depending on your budget a Gear Vendors overdrive unit will definitely give you a mileage boost and pay for itself pretty quickly.....
same for you Duramax guys......