In the last few months, I've become gear-obsessed. Not the expensive Garmin thingies, or more fancy bike clothes, but the type of gear that affects speed and hill climbs.
I've always been mystified by gear ratios. In fact, I've always been mystified by anything that smacks of math. However, the ratio can be expressed as gear inches. The formula for gear inches is actually pretty simple:
Note, though, that the drive wheel diameter should take into account the height of the inflated tire. So the diameter can differ depending on the tire used. I think. So figuring out the tire diameter, while not exactly a black art, is of necessity imprecise.
But my understanding of it is also this: gear inches represent a virtual wheel. So if I'm on my granny gear (22) in front and the smallest gear (11) in back, I'm running 54.6 gear inches. I'm also hopelessly cross-chained, but let's ignore that. So I'm running a virtual drive wheel of 54.6 inches.
In a real-world situation, let's say I'm taking a tough climb, or at least as tough a climb as I ever take. The chain would be on the 22-tooth ring in front and most likely the 26-tooth cog in back. That would give me approximately 23 gear inches, roughly equivalent to a single speed folding bike if you want to think of it that way.
While this seems somewhat uninteresting, the upshot is this: if you know how many gear inches you're using on difficult climbs, then you may want to know what kind of gearing you can use to potentially make those climbs easier.
And I bet you thought you were wasting your time.