If you don't want to replace the chainring then I'd get a 19T, gives about 73 GI and 19 skid patches.
If you're wanting to replace both then you can't go too far wrong with a 49 tooth chainring and a 17/18/19 sprocket.
19T, gives about 73 GI and 19 skid patches
21T would give about 65" which is more city friendly
I'm still looking for a ratio that'll challenge and build strength. But cheers still.
@gbj_tester started
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If you don't want to replace the chainring then I'd get a 19T, gives about 73 GI and 19 skid patches.
If you're wanting to replace both then you can't go too far wrong with a 49 tooth chainring and a 17/18/19 sprocket.