Originally Posted by old and new

Been riding for a long time. As obcessed as I can be about some aspects of bikes, gears #'s aren't among them. I'll just say that TOOTH per TOOTH, a much bigger difference occurs by increasing/decreasing tooth count in the back than the front.
A 39T ring gear as the smaller of the two in the front, isn't as different as 42T.
Going from 25T to 28T in the back is more of a difference, just for example. For the most part a greater range in gearing overall is available by changing the back.
My explaination..... ALSO search "bicycle gear inches" Google search, many calc. sites.