Well the Vapor, being the top-of-the-line Radar, is somewhat "designed" for short-line skiing at 36mph, or at least the size numbers are likely based on that. So my understanding is that if I was at 36mph and long lines, I'd probably use a 66", or if I was at 34mph and short lines, I'd use a 66", but being at 34mph and at longer lines makes me two "steps" away from the default size tuning for Vapor, and thus pushes me into a 67" size.