i have a 08 x unlimitrd with 17"

 

Without a spacer, about 10.50.
With a spacer of an inch, 12.50, 1.25", 13.00, etc.

 

i forgot to metion that i have 1.25 spidertrax spacers

 

You can get a 12" wide tire for certain on there with no backspacing issues and maybe a 12.5" wide tire. The stock 17s have 6.25" of backspacing and with the 1.25" spidertrax spacer you will be at 5.00" of backspacing. To run a true 12.5" tire the recommended specs are 4.75" of backspacing. Check the specs of the tires you want to run on your Jeep. Many tires are listed as 35s but in fact range in size from about 34 to 35.5", same issue with the width. If you are going to run a 12.5" tire you might have some rubbing issues.

 

I know a lot run wider tires than the manufactures recommend, but I'm that that sporty. If you had an accident where the tire separated from the rim and you injured someone, some jerk lawyer would have a field day if you put a 12.5" tire on a 7.5" rim and it was outside of the tire manufacture's specified limits. For example, BFG KM2s 35x12.5x17 are sized for 8.5" to 11".

My vote is to stick within the manufacturer's limits if you take your rig on the highway. Off road anything goes.

My 2 cents.

 

As much as I hate lawyers, I'll have to agree. With everyone suing for anything, a person has to be careful. How do you think they came up with some of the laws about tire/fender coverage? Height of head/tail lights? Bumper height? Mandatory mudflap usage? etc...