235/40-18 would be fine. 225/40-18 is fine too. You want a wheel with a 5x115 bolt pattern (although 5x114.3 will work too). Also, somewhere between 36 and 42 mm offset. Try not to go any wider than 8 inches, just to make sure you don't rub. Any wheel with the above specs will be fine. Go to
www.tirerack.com (use the link above to help the site) or
www.tires.com. Very helpful.