the stock size will keep your speedometer reading the correct speed. the rim width is only 7.5 on the front and 8 on the rear
so 235 front, and 245 rear would be the widest i would go without losing a ton of sidewall strength and having it stick too far out the sides. honestly i would save up the bones and get a good set of 18's. the info is also in the pinned section of this thread, under the FAQ.