I just got BBS RX 17's from the previous owner of my car, and the tires on it are 215/40. Two of them need to be replaced, so I went to the tire store, and they told me that to match factory size I need 235/40. Does that sound right? I'm itching to put the wheels on and dont know what to do. Options are: buy two of the 215/40, or buy four of the 235/40. Which is the correct size?