The original bike came with a SRAM XX 10-speed, 11-36t. I bought the bike used. The past owner put on a new Shimano Deore XT CS-M770 9 speed cassette before selling the bike. I'm kind of bummed that I didn't see this. I thought that it was a new SRAM XX 10-speed. Yes a new chain was put and and new bottom bracket but I don't know what brand. Besides the fact that this isn't as good of a cassette. Will it make a tremendous difference for a F high school racer? I'm mainly concerned that the gearing won't work together if the Deore XT isn't the correct cassette. and then I will have problems that cost me money and problems in the long run. I don't want to ride the bike if this could be an issue. I do think that I can talk to the original owner about this one issue. Eiither way I like the bike! Thank