I can't remember what the original sizes are (think it's 16/42), but I went one down at the front and 2 up at the back (so 15/44 me thinks). Makes it a lot easier to ride in traffic / commuting, and wheelies like you wouldn't believe (not recommended in traffic / commuting). Only down side is that it reduces the top end, but how often do you do more than 150mph to be able to notice?