Posted by: GiveitsomeWelly
This a little off-topic for this thread but I'm going up a wheel size on the rear (26" to 27.5") of my ss.
I'm aware that a tooth or two more on my cog (or fewer were I to downsize my chainring instead) will be needed to account for the bigger wheel.
Is there a loose 'rule' to how many teeth a wheel size should add to my cog if everything (including my trails) stays the same?
I'm currently coping On 32:18.
You can calculate what your new gear ratio should be based on gear inches: https://www.bikecalc.com/archives/gear-inches.html
Assuming a 26x2.25" tire and a 27.5x2.25" tire, 32:18 on 26" is 47 gear inches. To match that on 27.5", you could run a 31:18 (47 gear inches), a 30:18 (46 gear inches, slightly easier), or a 30:17 (48 gear inches, slightly harder).