This is one of those cases where the actual math is no more difficult than the rule of thumb.jesse said:I think the rule of thumb is for every tooth on the tranny it's 3 on the rear, so if you increase the tranny by 1 you would have to DECREASE the rear by 3 to end up with the same gearing.
It's a simple ratio between the teeth on the trans sprocket and the teeth on the rear sprocket.
Running a 21f, 49r combo . . . the rule of thumb is significantly off. Two and a third teeth changed on the rear equals a single tooth change on the front.
Just divide your rear tooth count by your front to get your present ratio. Do the same for any proposed changes to see where you stand.