Don't use a smaller sized tire for a spare, even for a short period of time using that 31 inch tire with 33's would cause more problems then its worth. As per the posts above, I would agree using a proper matching tire the best route. I've been dreading for the last year having a flat with the stock spare that came with my jeep. Tho the PO had put on 31's all around except for the spare which was a 30 when I bought my jeep last summer. Last month I had some extra money so I invested in a matching spare which will also be part of a 5 tire rotation.