Re: F550 mpg, gearing etc.
Excuse our manners!
Welcome to the best enthusiast's website on the internet. Hope you will get as much enjoyment and information here as so many of us do.
To begin with, a 250/350/450/550 are identical engine/trans (as you seem to be aware) and cabs. Only the frame/wheels/axles/springs set them apart. However, when it comes to towing, there is a BIG dividing line: 450/550 have different brakes from 250/350, and the difference is considerable.
The 450 (earlier) has a Dana 80HD axle, and gears, diffs, axles, etc. are widely available. '99 - '00 trucks came only with 4.88s, and I thing it was '01 when 4.30s became available, so if this is your choice, look for the ratio or be ready to spend the $$. I drag 24k tailer all over on 4.30s, PLUS 0.8555 OD, so you don't need 4.88s for anything. It is a dumb ratio. The 550 used a Dana 135 (much, MUCH heavier) in the earlier years, and it is nearly impossible to change gearing or differentials.
The 7.3 is a lot more proven, and for moderate towing duty (say 300 or so HP) it is pretty good. The 6.0 is just a 4 valve engine using the same obsolete injection technology (CAT HEUI) which has been...er...a little less than stellar in performance and reliability.
You must be VERY careful about clutches and pilot brgs in ZF6, or they will take out the gearbox.
My recommendation: get the latest model 450, 7.3, 4.30s with Dana 80HD you can find, &/or wait until the new 6.4 with HPCR is available and proven.
Regards - Pat
2000 F450 Lariat 4x4 tractor, rebuild at 200k in '12, Air Dog, Reg Mod, HX & HP all in hard SS lines, bypass filter, Banks Power Elbow w/3.5 dp, no intake heat, CCV mod, AIS (w/extra hole), some big Alliant single shots, GP indicator LED, custom 4" exhaust, 4" USG exhaust brake, custom alum. rear diff cover extension (dipstick & temp), Detroit locker, ESPAR D5 aux. heat, custom alum tow deck and fuel tanks, Bushwacker 1 1/2" flares, 9k winch, gin poles, JWSpeaker 8900 LED headlamps in XL grille, Lightforce 240GT (still Halogen, but VERY bright), Hella Micro DE HID fogs, KW = vision plus (Isspro) instruments, more to come & lots of other diesels