I have a 94 F250 long bed, supercab 4x4 with 460, auto and 4.10 gears. I get almost always around 11mpg for normal freeway and around town driving. If I tow my 8,000lbs + trailer I get around 8 or 9mpg unless I am in heavy hill climbing. I have some times seen 12mpg on a good day, no trailer, wind to my back, but can only dream of 15mpg.

A 350 should get a little less mpg due to more weight, assuming all other factors are the same. Different gearing and tires will affect the mileage.

Yeah, I have also heard that while towing the diesel mileage gets down near the gas. But with no load on the freeway it is up in the high teens, around town down in the low teens. The diesel appears far more sensitive to stop and go and maybe loading than the gas. Ie gas mileage is poor almost al the time, diesel swings a lot.

Good Luck,

Jim Henderson