So you're going to replace the 15x15's with something like an 18x15? That's like a 375mm+ tire back there. I can't even find a street tire on tirerack with that width. I think you're going to have to look in the 315-335mm width range for a street tire and a 12" wide rim, and almost no offset to push the rim outward. I can hear the cash register ringing already.