Just wondering but are you sure thats what they are? Ive never seen a rubicon come with 18" wheels only the saharas. As for size though you shouldnt have any issues at all. Although if you like the wider stance a good set of adapters never hurts and i always suggest 1.5" minimum for log clearance. Ive seen some people get away with the 1.25" adaptors but others end up having to grind their lugs or the wheels wont sit properly and you become a rolling hazard.

 

I am currently putting on a set of 18's with 255/70 r18 off a sahara onto my unlimited x. No issues at all. They are stock rims with michelin ltx's. As long as you dont go any wider then 10.5 inch you shouldnt need a spacer. a good tool for to get size difference is Tire Size Calculator - Compare Tire Sizes This was sent to me from a fellow jeeper and was a rather nice tire comparison calculator. 11 inch wide tires will need at least a 1 inch spacer. the wider tire you go over 10.5 inch the bigger the spacer that is needed.

 

Also on that calculator is an estimate what it will do to your speedo. i do believe on 16's the stock size is 225/75 r16.

 

ok for some reason my first post didnt get posted. Any way I am doing same thing got a set of stock 18's off a sahara to replace my 16's on my unlimited x. No issues at all as long as you dont excede 10.5 inch wide. I ran them on tire calculator and they are 10.4 inches wide. A good tire calculator online is Tire Size Calculator - Compare Tire Sizes. That site was given to me by a fellow jeeper and worked wonders. Also as my first/second post states it also gives you how much it will effect your speedo.

 

I am 99.9% sure you do not need spacers. My 285/70-17 barely rubbed at full turn, and you tires are 0.20 inch shorter, but most importantly, 0.75 inch narrower. So it should not rub.

 

Rubicons have never came with 18" wheels.

Regardless, you will be fine with no spacers. They are pretty much a stock size tire.

 

The wheels turned out to be of off a Sahara , the chick who owned it, did not know that