[flashgordon13]

In theory you should replace all 4 tires, but in the real world it will make no difference if one tire is not exactly the same diameter as the others. Just ask what the manufacturing tolerance is on new tires and you'll get a blank stare. The truth is that even brand new tires are different diameters. That is why race teams have guys that spend hours measuring tires to match up sets that are close to the same sizes. I would just throw a new tire on the car and keep cruising.