Hey 4x5,
When I put my 31x10.5 A/T's on I kept one of the 30x9.5 A/T's as a spare and it fits like a glove underneath.
Well 2 weeks ago I punctured a sidewall on my 31x10.5 and had to ride the spare. I was skeptical that the 31x10.5 would fit underneath so I just threw it in the back. Well after getting home from the trail curiosity got the best of me, so I attempted to put the 31x10.5 underneath. What do you know, IT FITS!! You just need to guide it past your rear swaybar, then it snugs up in there quite nicely!
In short, 31x10.5 is the largest I have fit, and probably largest you can fit.
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Cheers,
Andrew B.
1996 Ford Explorer 4x4 (slightly modified)
BC, Canada