The rear sprocket is 41 tooth, the front is 15 tooth on a stock bike. I bought a rear sprocket from intobikes.co.uk (a 44 tooth Renthal) for about $50 shipped. The people from intobikes were responsive and helpful- my package is on the way and I have a tracking number. I went with the 44 tooth for more acceleration. Check out the site, they probably have smaller rear sprockets for lower rpms too.