The outside diameter is almost identical...with the 20's you get larger wheels and lower profile tires. There is a VERY small difference...with my '05 Lariat I upgraded from the 18's it came with to a new take off set (from an '08 F-150) of 20's...did the calculation and it was a less than 5% difference, IIRC. I immediately noticed the '05 drove more precisely and had lost much of the "tire wallow" that you feel with the taller sidewalls and are magnified by a truck's suspension. The truck also handled better and the ride was a tad stiffer. Well worth the overall improvement in my book. My '09 Platinum has the 20" wheels/tires and they ride even better than the 20" wheels did on my '05 lariat...could be that the overall suspension is improved and/or that it is now "tuned" for the 20" wheels/tires. In '05 20" wheels were not an option. The big draw for me though was the 20" wheels look soooo much better than the 18" (IMO) and I have such bad luck with aftermarket wheels, I wanted to stay stock (no tracking or uneven wear issues, etc). Get the
20"s now if you are going to do it - aftermarket will generally be more expensive if you wait and you won't have the stock ride either. If you settle for the 18's - my bet is you will regret not getting the 20's and it's going to bother you every time you look at your truck!

 

I could be wrong but I think this only applies to the 09's n up. I don't think the 04-08 body style's received and extra leaf b/c the factory installed 20's. My truck came with 20's stock and I've never noticed this

 

i opted for the 20's because they just plain look better, and with the 285 55 20's BFG T/A's i put on my FX4 they look even better then my 04 did with 18's with the same tires.

there are pic's in my album.

 

If your truck is going off road and will go though places with cactus and mesquite trees, you will need an LT rated tire, not a P rated tire. BFG All Terrains and some models of Michelin are the most puncture resistant. You can get LTs in 18" (not as available as in 17") but may not be able to get them in 20." Short side-walled, P series tires do not do well in the rock/cactus/mesquite county. Everything is a trade off of some type.

 

Depends, are you talking 4x2 or 4x4. With 4x4s Sidewall is king off road... You need the flex that sidewalls offer. Taller rims steal away from that. Not to mention the appearance issue. 20s are MUCH more appropriate on a street truck or 4x4 pavement queen...

 

I opted for the 17s on my FX4 since I will use it for what it is designed. Spend a little time offroad. The 20s on the 2009 (at least the FX4 package ones) come with sport suspension so that makes a difference in the ride. If you want to lift it you need the 20s do to issues with the lift kits and the new design (BDS is the only lift I know of that keeps OEM Geometry and allows 17" wheels). Also the stock 20s are 8.5 wide vs. the rest of the line up which have 7.5 wide rims.

So do your research and decide what you want and how you plan to use the truck. But offroad side wall is the king and you have to realize that the tire is a shock absorber also. The more tire (sidewall) you have the more the tire can absorb the bumps from the road.

 

There are lots of LT load rated tires for 17" and 18" wheels and several for 20" wheels. Maximum sidewall height is only important for higher speeds off roading, or potholes on roads.

Get the wheel that looks the best for you, from design, material, and proportion of wheel to tire. If you plan on replacing them with something aftermarket, then get the cheapest OEM wheel.

 

You've obviously never been rock crawling, which is not real likely to be done with a brand new pickup, but... larger sidewalls, and running at lower PSI works very well for allowing the tire to wrap around obstacles, allowing it to get a better grip on the obstacle, be it rocks, fallen logs, or what have you.

20s will work fine with larger diameter tires, but in more "normal" tire size ranges, the shorter sidewalls can be a real disadvantage. Especially when the going gets tough.

 

1.5" ((20"-17")/2) less sidewall is not going to be noticeable to 95% of rock crawlers.

 

Not that anyone is even going to be doing rock crawling in a 09 f150 lol

 

I drove both before ordering (in snow both times) and the Pirelli's weren't very good at all. I went with the 18" w/the AT's.

 

32" or 33" tires on 20" rims still have a considerable amount of sidewall. I've run 33" tires on 15", 16" and 18" and prefer the shorter sidwall. It would be a different story on a mustang. What I typically do is get the standard wheel, unless there is a factory wheel I really like. Most stock rims are too narrow for wider tires, so an upgrade for aftermarket rims may be the thing to do anyway and it doesn't make a lot of sense to pay the OEM price to get 20s" for wheels you may end up replacing. Not that I had a choice on the 2008 I picked off the lot, but I will be installing 20"s when I upgrade to 33"s.

 

I went with the 18's because when it would be time to change tires I couldn't justify the extra cost of the 20" rubber.

 

I wanted to test drive an XLT with the 20" wheels. When the salesman pulled the truck around to the front of the dealership he made a wide u-turn and slightly scrubbed the curb. Since there was very little sidewall to take any of the curb, the wheel took a significant scrub. Bent the lip of the wheel pretty badly and scraped nearly half of it. Wasn't really an impact at all but a bit of a glance. I decided right there no 20's for me on a 2wd truck.

 

People will be rock crawling in 09 F-150's. Most of them won't do it until 2015, but some will right away.

v_tach, any OEM wheel probably would have gotten curb rash. It's usually a matter of how wide the tire is vs. the rim width. The more the tire hangs over thew rim, the less likely to get curb rash.