I've never gotten this to work on my work computer, but a few quick pages reveal to me that MacG is, in fact, a genius:

http://www.basementfreaks.com/members/karl/gearing/

I don't think there's much that you could be looking for that's not in that calculator/table...

[edit]And I finally remembered to bookmark it.

benk0 :

49x19, 23c

67.83 inch
19 skid patches
99 rpm @ 20mph
21.2 mph @ 105rpm
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