I usually fill up.. same gas station all the time, different pumps, and i make sure i fill it past when the pump click off. I didn't think it would swing that much.. i usually MAX sway .5-1 mpg different.
we just got rid of our 06 explorer with the 3v 4.6 and best average was 23 with me driving highway...and my wife averaged 18.5 from tank to tank mixed driving...the v8 also came with a 6 speed auto unlike the v6 that has a 5 speed. Our friends 06 eb v6 averaged the same as ours.