What are you running now and how do you like the performance? If you're running a 14, you can probably bump up to a 15. It all depends on where you want the power. Going higher you'll lose a little bottom-end power (will lose over full RPM range, but most noticeable at low RPM), but will allow you to hold a gear longer and reach a higher top speed. You can order one from Rocky Mountain for about $10.