I have 3/32 tread worn off my 33x12.5's. My tires are Pro Comp X-Treme AT's, they are a directional tire, I know I can only go front to back, not side to side. Here is what I'm wondering, I have a brand new spare, can I put it on the back now? I will work it in the rotation every 5-7K's, actually break it down once it is ready to go to the other side. Should 3/32's make a difference, will it screw up my gears?

 

For a full time 5 tire rotation, you'll want all 5 to be somewhere close to the same treadwear. Get it in as soon as you can, and keep the spare in the rotation every time. (PC wants the tires rotated every 3-5k, documented, for warranty purposes)

For a 'get you back home' spare, you won't need to worry about the tread difference.

If you are driving with an auto-locker, you'll want to match up tire heights, and put the closest pairs on the same axle. Open diffs, a small height difference shouldn't matter. (I did see one post indicating that it may have caused a wobble, but haven't seen any others to back it up. Might have been mkjeep? Or maybe planman? Been a while, just don't recall who...)