we had a 1996 F250 460efi with ZF and 3.55s. had 240,000km and only had it on the road for about 2 yrs. nobody would drive it because the rules were you had to fill the tank when it came home. fuel economy was ALWAYS between 5 and 7mpg. anything from empty trying to shift at 1500rpm to save fuel and hauling 8000lbs behind it. this truck is a 2wd CCshortbed . so in the 52-5500lb range empty. it was horrible , dual tanks were not enough!
the truck got parked !
replacement was my E350 extended clubwagon 7.3 powerstroke 4R100 , 3.55s it had 360,000km on it. empty mileage is 18-25mpg depending on ambient temp. trailer towing 16-22mpg . it weighs 7000lbs empty with a handfull of tools and a coupel chains on the rear floor. I drive it hard empty or loaded. and have hauled over 12000lbs behind it many times. it is my daily driver also.
the truck we had before the 460 crew was a 1990 F350 supercab dually 7.3idi ATS turbo,e4OD 4.10s on the little dually tires. doign the same work it could get almost 20mpg on a highway run but usually 15-18mpg empty or loaded. sold it to a buddy who consistently gets over 20mpg" he drives truck for a living so different long skinny habits than me.
we pay a lot more for fuel here in Canada so mileage is a big factor
if you are only going to put as few thousand miles a year on it go big block. my diesels have not been much more expensive to run other than oil changes because the diesels get about 3 or 4 times as many miles put on them.
a 300 6 cyl efi with a 5spd manual can make some fun for you with some minor mods and still haul a trailer when you need to.
if you want some fun with a 460 , build it 12-13:1 compression
and run propane. here in Ontario propane is half cost of 87 octane.