We got my son a small youth atv, a Kayo Storm 150. Stock tire sizes are 20x7x10 front, and 19x10x9 on the rear. We've ridden a few dirt ohv trails and it ran great, we took it out to the dunes and we had trouble getting up and over any hills, his tires just spun out constantly so I'm looking for some paddles for the rear. I can't find any 19x10x9 tires out there (Looks like a 250 Raptor ran that same size, but can't find those anywhere), the closest I've found is a pair of ITP Sand Stars in an 18x9.5x8 setup, or a 20x10x10 version. I'll be getting a 2nd set of rims for whatever we go with so that isn't a big deal. I'm very unfamiliar with tire sizing and if going up to a larger tire would be an issue, and vice versa on a smaller tire.