I ran a spare that was 31", but whell worn for about 20 miles... it really effected handling. With the missing tread, it was about 2" shorter. But I have open diffs. Ive also tried putting the tire up on the roof. 2 things about that. 1, In PA< unless it is secured to something that is not the sliding factory roof racks, it is not OK. Was told this by an inspection guy, something about those tiny pins not having the oomph to keep it in place in a collision.
2nd, it will effect your handling, it feels like if you were trying to run full tilt with a very , very heavy helmet. Or tryng to run with a sail mast... If you do not need your seat locked in all the way, you can get away with just mounting it in the factory location (I used a ratchet strap) and not letting the one side of the rear bench to latch. Pu t a cloth on it, as it will squeak against the hatch plastic.
(3rd crappy thing about the wheel on the roof... getting it down. IT sucks. Its heavy, and you will probably have to do it, when its raining or snowing, and you have to be somewhere and look half presentable. Oh, and if you guys with 96's and older think about this, that time you spend up there on the edge of the hatch? it will embed itself in your arms, and you will itch for a week. I know.)
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