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.