Sipe them and they'll do better in the rain, but I don't think any swampers are that good on wet pavement. As for asking how long they'll last, that's only up to speculation. 20,000 miles maybe if properly rotated and inflated, maybe more maybe less. It's just too hard to say since 2 rigs set up the same can wear tires at a different rate. Figure on getting roughly 1/2-2/3 the miles out of them that you get out of your normal sets, but again, that's very rough.
