Originally Posted by
Litespud
No, it's Pi (or 3.14, as I stated) x the diameter.
If one has a basic 700c wheel, then I typically ignore the wheel size, and just consider gear ratio (front divided by rear).
If you are experimenting with different wheel sizes (20" mini velo or folder, 650c, 700c, etc), then one might go ahead and add in some kind of gear inches calculation. Although, one could also simply multiply by a ratio of wheel sizes (diameter or radius). pi drops out. And one gets an adjusted ratio.
Of course, Junior Racing goes by their own set of rules and rollout is ever so important which one can calculate, but at the race one will need to measure.