I owned a couple of F350s with the 460 engine. Neither ever got over 12 mpg empty - and 8-9 mpg with a camper. Both had 4.10 rear ends though.
Ya,I sure didn't expect that coming from another 460 in a small class c that did the 9-10's,but I'll keep it..My sons boss has an old 460 he claims does 14+ on the highway..I can't get that..I have just never been lucky with the big blocks and gas mileage until now..
I had a slew of 454's..On one,when I hit 150 miles there better be a gas station shortly pulling a large trailer..Another seemed like it got 8 no matter what I did with another dipping to 4-5-ish mpg towing,about the same as my old Kenworth.
One just never knows..
Example:..In high school the fastest car in town(Boise Idaho) for the 1/4 mile was my friends dad's car..Chevy Malibu with a stock 396/automatic.. Beat all the hopped up cars in Boise with ease...It was a one of a kind..So every once in a while a really good engine comes off the assembly line and most everyone thinks there all the same,but there not, for some reason!
I have learned that when towing heavy or hauling heavy,mpg differs sometimes by quite abit with the same factory marked engine because of driving habits...To some it's the petal to the metal(The need for speed and bragging rights) and others don't really care and just enjoy the ride with there RV..
As to the silly remark earlier that speed don't kill...You couldn't be more wrong!