The 12x23 is ...12,13,14,15,16,17,19,21,23. The 12x25 is 12,13.14,15,17,19,12,23,25. The recc is bogus.What big cog you need depends on how strong you are, and whether you need the lower gearing. Some might do the course on a 11x21.Make sure the chain is long enough to accomodate the bib/big combination.If the has been on the original cassette for some time, its usually good to use a new chain with a new cassette.