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What is the closest range cassette I can use with a long cage mech?
Which long cage? There's a lot of variations. The good news is that the first 4 of 5 sprockets won't make much difference, the shifting only really goes to shit if your big sprockets are way smaller than the RD wants. Back in 9-speed days I used an RD which wanted 32 or 34 as biggest sprocket and it was fine all the way across an 11-23 cassette, so there's that. And it was SRAM, Shimano RDs with a floating B pivot will be even more tolerant of out-of-spec cassettes.
What is the closest range cassette I can use with a long cage mech?
I have a tourer with (I think) MTB gearing (possibly 3*9, it's not new) and I spend most of my commute in the big ring and bottom 3 at the back.
Thinking of swapping the cassette to a closer ratio (or swapping to a road double chainset is the alternative)