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If I use a 34t narrow wide on a 27.5, what would be the nearest equivalent on a 26”; thanks
Assuming the same cassette ratios then 36t. But the difference in wheel circumference between 27.5 and 26 is so slight I doubt you would notice if you kept it the same
In "theory" it would be a 36T but it depends on the circumference of each tyre - I've seen 27.5 tyres that are no bigger than a 26" next to it whereas others are noticeably bigger.
Generally subtract two teeth for each jump in tyre size: so 36T on 26" is equivalent to 34T on 27.5/650b is equivalent to 32T on a 29". It's not exact but it's useful as a starting point.
http://sheldonbrown.com/gear-calc.html
Gear inches varies with tyre size, even with the same wheel rim size. Swapping between 700x28mm and 29x2.35" tyres shifts the gearing by about half a gear on my FatNotFat wheelset.