My 2004 250 4x4 v-10 avgs 16-17 on highway the best it has done is 17.8 highway, Towing about 7000 it gets running 60-65 it gets 12 and that includes hills. My truck does have the air box and front grille cut-out for more air-flow, the off-road y-pipe and a magnaflow duel exhaust with 18in wheels

2005 f-250 harley davidson crew cab

v10
kn cold air
flowmaster catback
33" tires
3" lift

9 dicken around
10-13 on the highway
7-8 towing 10,000lbs

the air filter and exhaust was worth about 1.5mpg