Rim offsets on 07 f-150
#18
You should have no problems running a 33" tire with those wheel specs.
Compare my 325/60/18 with your 305/55/20
http://www.rimsntires.com/specs.jsp
#19
#20
Yeah I'm actually considering going to a 17 inch wheel. I like the look of more tire then wheel plus I'm not really happy with the XD Monsters I got a few months ago. I hate the center caps on these wheels. I'm going to machine some custom aluminum covers and see how that looks before I ditch them. The wheel I really like is the EA 178 in chrome but they are discontinued.
#21
#22
I would guess that a 33" tire with 20's would weigh a little more then a 33" tire with 18's but I doubt you would notice much, if any, in fuel mileage. Other than that, 33 inches is 33 inches. If you're trying to save money 18" wheels and tires cost less but if your heart is set on 20's then get them. If your spending $2k +/- on tires and wheels you might as get what makes you happy even if it costs you a couple of bucks more initially.
#25