8/9/10/11 speed? What gear range is the existing one? Mountain or road cassette?
I generally get cheaper cassettes and chains, because they all function pretty similar and its a wear part that I feel weird spending a lot of money on. I’ll go to the lower mid-range stuff and see what I can find on sale in the gearing I want.
For compatibility, you want the same number of gears (obviously) and to make sure the derailleur can handle the cassette (a short cage road derailleur won’t play nice with an 11-36 mountain cassette, for example).
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