I have a 15/39 tooth sprockets right now, and i like it. i don't ride to much highway, but lots of city and country riding. my top speed is 160 on the GS speedo, the sigma's battery is dead so I can't give you an exact value. the bike pulls much better than with 16/39 stock, but if you do lots of highway riding, I wouldn't do it. 15 tooth sprocket costs around 15 euro's in holland