31s will be the same size no matter what size rim you have. If you have stock 31x10.5R15 a 31x10.5R17 will be the exact same size tire. A 255/70R17 would work out to be a 31x10 which would be slightly narrower than your stock tire and a 265/65R17 would be a 31x10.4 which is very close to your a stock 31x10.5R15. Any of those 3 choices will keep the same amount of space as stock in your wheel well as well as keep your speedometer accurate.They should fit or will fit? Have you ever put 31s or 32s on 17's before? I should also add that most of my driving will be on hard surfaces, not on back woods trails. I do want a little bit of aggressive looking tire though.