LIsurfer_

Will the difference between 18 rears and 22 fronts be ok?

I am looking to replace the tires that were on my shee when I bought it. They wre 22x12.5x9 ITP blackwaters on the rear and 22x7.00x9 puresports on the front. I think they are on stock rims. If I change just the rears (the fronts are near new and I want to spend my cahsh on other upgrades). Can I run 20x11x9 Kenda Klaws on the rear? Does this set-up sound ok to start MX'ing on? Also what chain/sproket config should I run on these tires? The shee is stock. Thx 4 ne help.