I've owned my truck for 25 years now, so I have it's tendencies down. It originally came with a 360 V8, which was later changed for a 390...no difference in mpg, maybe the 390 was slightly better. The 351 you have is a decent motor on gas, better than a 390 is.

The truck actually came with 7.50 x 16 mud and snow tires on split rims (which are no longer legal, due to some terrible incidents during changed tires at some tire shops). I have 2 sets of tires for it 35" BFG's and LT275/85R16E radials on solid rims off of a late 80's F350 4 x 4. The 35's do get a litter better mileage than the smaller tires do.

Overall the mileage is fairly consistent, what hurts me is that my truck sits to much. The more I use it, the better it runs and the better mileage it gets.

I think that if you can get between 12 to 15 mpg highway, you are not doing bad. It's not a Honda Civic Hybrid!