I have seen near 20 mpg with my 7.3 n/a ZF 4:10s, but that's at no more than 62 miles per hour with no load. When I'm in a hurry going to Seattle and turn it up to 75 the mileage routinely gets cut nearly in half.
Careful driving and maintenance seem to make a huge difference in the IDIs.
Careful driving and maintenance seem to make a huge difference in the IDIs.