Originally Posted by
jhhall
While I don't use this combo often, I do use it in certain situations.
All you have to do is find a similar ratio with the large chainwheel. 34/12 is 2.83. Just divide your large chainwheel teeth by 2.83 and that will tell you what rear cog gives you the same ratio. Absolute worst case is that it will be off by 3% or 1/2 of a 1 tooth jump.