Check out the fordsix.com forums: more info on that engine in one place than any other place Ive found. Consensus there seems to be that mpg tops around 20, but thats without any aeromods. There is a guy named lasitter that has done a couple aeromods as well as improving the efficiency of the engine that claims over 20mpg. I believe he has a 96 w/ MAF so maybe thats the ticket. I have a 84 f150 4x4 with the 300 that was getting 18-19mpg on the highway before it broke a ring and tore up a piston; hasnt been the same since
It has manual w/ no OD and 3.08 gears, stock p235/75r15xl tires. Driving style/type has a big effect on mpg w/ a fullsize truck; If you arent willing to change that, youre going to be disappointed.