A lot depends on the real size of the tire not just the markings on the side wall. If its a true 31" tall tyre I think you'll find it will rub, the extra diameter is as much or more of a problem as the extra 30mm in width. My advice would be if you want a certain size, get the same brand as someone else who is alreay running that size, or if you want a certain tyre make/type find out what size other people are using it in. As for what sweety said, remember the wheel moves up and down a lot especially when laning or off road so a quick static look isn't enough. If I'd have bought 31" versions of my tyres they would not fit as theres barely enough room for the 30s, other people have got 31s on of different brands with no problem. I think the cheaper brands and remoulds tend to be more optimistic with their sizing than the new branded tyre manufacturers. Mine for instance are Yokohama's, no point looking for any they stopped making them some time back, a good match is a Mach, but I'm not sure what their sizing is like.