I have a 26" 24 speed (3 front, 8 rear) and it's all Shimano equipped. I ride for intense exersize like up and down city hills. I never use the front small 2 crank rings, always just the 3rd. Should I just upgrade to a 10 speed cassette? What all is needed to convert to such? The cassette, rear shifter? Does the derailleur need to be changed as well? Is there anything else, or any other way I could pick up more speed down hills? Right now I max out at ~42mph on the biggest hill in town and I can't pedal fast enough to keep up with it. I can really only pedal to ~38mph effectively. Thanks.