I just bought a set of OEM 20" wheels from a guy on Craigslist to replace the 17's on my 07. He had them on a 2006 FX4. I asked him about buying the spare as well so I wouldn't have to go through the trouble of finding a replacement one for mine. He tells me that the spare is an 18" rim.

Did Ford put 18" spares on the trucks with 20" wheels? Or is your spare a 20" rim as well? And if it is a 18, is the overall tire diameter the same as the 20"?

 

My 06 came w/Perellis on 20's and the spare is a Wrangler on an 18

 

What size is the 18? And can I do the same thing with my 17's?

 

As long as the overall size (diameter) is the same it doesn't matter. What are the sizes of your tires?

 

Well thats what I figured yetti, however, its not always an exact match. I plan on going up to a 305/55-20, what would I have to do to have a functional spare besides buying a fifth 20" rim and wheel combo?

 

Try this link...it shows numerically and graphically what size tires compare to others. It is a calculator so you must enter in the sizes.
http://www.miata.net/garage/tirecalc.html

 

How much of difference is allowable between the tire thats on there and the spare? For instance, lets say the spare is a 1/4"-1/2" off in diameter. Is that going to screw up my differential and the gearing?

 

My spare is a 275/65/18 Wrangler ATS
My Scorpion ATRs are 275/55/20

Both are approx 32"

 

Thanks bro, thats the answer that I was looking for.

So basically,

275/65-18 = 32.075
275/55-20 = 31.909

Thats only a difference of .166

I noticed that the widths of the tires are exactly the same but the sidewalls are different. Does the width have to be the same?

If I'm going to be using 305/55-20's = 33.209, can anybody recommend what size spare would be the closest match?

I know I'm really dragging this out but any help would be appreciated.

 

As for a tire that would fit your 17" to make a spare close to a 305/55/20 I don't know how close you need to be. If you look at some of the ones that come with cars they are small compared to the OEM tires.
But the closest I can think of might be a 285/70/17, it is 32.76"x11.69.

One think you might have to think about is will the bigger spare fit where the old tire did.
I don't know the answer to that.

Get wandell's attention from all the posts I've read he seems like the guy to talk to and can probily answer all you questions.

Good Luck!