Count the number of "gears" in the front and the back and multiply them together. So if you have 3 in the front (which more the likely you do) and 7 in the back you would have 21 "speeds". I am not a bike mechanic so I don't know if you would have to stay with the same speed or not. I am sure one or more of the great mechanics we have on the forum will chime in with good advice and let you know if you can go and get a 9 speed rear end or not.