Your oem 225/40 18's are roughly 25 inches in diameter and the 255/35 19's are roughly 26 inches in diameter. The 205/55 16's are fine for the front at roughly 25 inch diameter. However, the 225/50 16's are also 25 inches in diameter and way undersized for the rear. Go to www.tirerack.com and check out the specs on the tires. My guess, and it's just a guess, is a 235/55 16 might be 26 inches in diameter. Happy hunting.