When I had my 31 strapped to the stock rack on my old '01 I noticed no decrease in MPG. On my current rig I plan on going with the rear bumper swing out carrier as I want to keep the rack free for camping gear. Right now I have my 31" spare mounted using the stock stud, I removed the metal bracket the stock spare sat on and ran the stud thru a lug hole and secured it with the Jeep plastic wing nut, seems to work ok, the top of the tire does hit the left rear speaker though.

 

My 31 does on the stock rack does not allow the rear seat to lock on open position

 

I've heard this from others and just can't see it. I'm not saying you're lying but I put 4 32's standing in the rear with the seat up and had no problems. Like I said, I'm not calling anyone a liar I just can't see it not fitting.