4BT's rarely get over 25mpg in vehicles that large due to weight & wind resistance. The reportedly "high mpg" (upper 20's & low 30's) rigs tend to be 4500lbs or less, 2wd w/5 speeds & road gears. 20-25mpg would be a good estimate in that particular application which would be nearly a 40% improvement over a 5.4L that might average 12-15mpgfrom what I have found curb weight is 5970 on the navigator and it is on the f150 chassis instead of the super duty chassis. My 99 super duty weighs almost 8k with a 7.3 it gets 16 and has seen 19mpg before but not often. I have a 97 powerstoke also and with no spare tire, and me out of it it weighs 6500 at the local scrap yard and it gets a consistent 17.2 mpg with 4.10 gears and a manual.