If this is a dedicated track bike for a roadrace course, the go as tall as possible. I have a 06 450X that's running 15/43. Nelson Ledges and Mid-Ohio are fast tracks and this works ok, but some fine tuning is needed. At Nelson I'm between gears on most of the turns and just ordered a 40t JT. Go with a steel sprocket, you'll really rack up the miles on a RR course. You didn't say if you have an X or R, any mods or running 17's. The R is hotter, but the X has the WR tranny. My X is opened up and rejeted. I'll go to the R cam next year for the best of both. Consider having the suspension revalved, the dirt set up wallows too much. Factory Connection did mine, very happy with it. Drop the triples 5mm and if you have bars w/o a crossbar try flipping them over and backwards. Takes a little getting used to, but puts you in the same position as clip-ons, but much cheaper, this really plants the front. Just don't get discouraged on the staights, because everything short of a Ninja 500 is gonna walk away from you, But the first time you rail outside of a Gixxer liter bike and see the riders helmet snap over to you, it's worth all the time and money and just laugh as he repasses you on the next straight. Anybody can go fast in a straight line.:laughingr