Being uncommonly common, I have never really checked it out. A big part of my driving is on rough back roads though. I think I get around 15 but not really sure. Hopefully, if the weather cooperates and all the snow is gone and the roads are free of the killer salt, I will find out sometime, alas, prolly not before April though.
I never saw close to 20 when running my 223 (3.73 3spd. stock tires). But I always believed there was something wrong with it. It never had any power either. Did rings & valve job & timing chain. Checked cam lift & timing...never happy. Swapped for 8cyl & OD never got mileage but happy w/ power.
I was able to get real close to 18 but that was with the smaller diameter 1b carb. Since I put the larger non-mileage-maker 1b carb on it seems to have dropped down to about 16. I do like the little extra power though. both with 4.10 gears and non OD tranny's.
Edit: most of the roads around here seem to be old Ford friendly because the speed limits are mostly 45-50 with the occasional 55.