The late Sheldon Brown’s web site is still THE resource for understanding the mystery that is gear inches (as well as a lot of other bicycle-related facts). What are gear inches ? It’s a fancy way of figuring how much a given combination of a bicycle crank ring and a cassette will move you with a full turn of the pedals. (I am really simplifying!).
Why would I care ? Well because I have a bike race on Sunday and the last time I was out riding in a group with a tail wind, I couldn’t mash the pedals fast enough, even at 125 rpm, to keep up. I was using a cassette with between 12 teeth and 25 teeth. I have another one, a much friendlier one for flat land, with between 11 and 23 teeth. I wanted to know if the difference between was significant enough for me to want to unmount the wheel, pull out my chainwhip and other tools, and change cassettes.
Sheldon Brown’s gear calculator told me that I would be about 10% better off with the smaller, more aggressive cassette, in the toughest gear.
The first chart is the easier of the two cassettes:
That being said, the question is more along the lines of how often I expect to be riding in the highest gear. Since this is not a time trial, but a more bursty road race, it may not make as much sense. More likely than note, I will be riding in the middle of the cassette, as per usual. And this is a hilly course, so the extra easier gear at the end (25 teeth instead of 23) will be appreciated.
I still have most of the evening to think about this. So I’ll give it some more thought.