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@gbj_tester
It's 71.4 on my chart, but I think my chart actually underestimates the true rolling diameter, partly by not accounting for tread band thickness and partly by being optimised for 23c on 20mm overall width rims. Using a wider tyre will increase rolling diameter slightly more than twice the increase in sectional width whereas my chart just adds twice the sectional width to a constant base circle. Increasing rim width will reduce the rolling diameter slightly for any given tyre.
I know I usually have to fit 20mm tyres to make 49/18 squeek under the 18'10¼" rollout for medium gear events. This year I'm going for 48/18 and 25C.
49/18 + 20C is 71.4 on my chart and actually rolls out very close to that (i.e. about 2" short of 18'10¼"), 48/18 + 25C is 71.0 but allowing for the known errors I'm not expecting to be any shorter this year.