Re: 7.3 info
Rory, 10-11mpg while towing a fifth wheel isn't too bad. With a 5.9L V8 (gas) towing just 5500lbs my towing best is 9.4mpg! A friend of mine with a cummins QB SB 4x4 5.9L diesel with auto trans gets 13mpg average when he tows. FOr the difference in displacement and the fact that Ford trucks generally weigh more part for part 10-11mpg with your 7.3 doesn't seem to bad.
One thing however that could go either way is a performance chip. Most have a towing/performance setting that will up HP and TQ by a good 20%. Most also have another performance setting (non towing) that'll bump HP by nearly 40%! I haven't heard any real mileage stories where they say mileage does increase, just whats printed in the tabloids. Cheers.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!