shoe size =/= boot size. depending on how you wear shoes, you should go down at least one, sometimes 2 sizes down. I wear a size 10 shoe (about 28 mondo) and a size 26 boot (about size 8 shoe) fits me perfectly for the most part, toes touching the end, easing off as i flex. If you wear size 13 shoes, that equates to about a 30, so i'd bet for ski boots you'd fit pretty well in a 28, 28.5. granted, full tilt sizing is a little bit funky, so you might need to go up to a 29, but that's still a pretty big boot. this is pretty general, and without seeing your foot there's a solid amount of speculation here, but a 31.5 is one hell of a big boot, like a size 14 or 15 shoe, reserved for basketball players and pornstars. i'm sure they're comfy, but i find it very hard to believe they'll hold your foot properly.