If you look carefully, you see that most if not all offroad bikes have 18" rims still, but motocross bikes have changed to 19". I think there was an article in DirtRider mag awhile ago on this, or on this site. The outer diameter of both the 18" and the 19" tires are about the same. The 19" rim tires have less sidewall though, so accelerating out of turning, turning or something that is better or more precise with the 19". The 18" is good for offroad because there is more sidewall, making it harder to bend you rim on a rock.