How to use:
Define wheel size by (A) choosing ERTRO/ISO rim diameter and (B) tyre width. The nominal widths are 25, 28, 32, 35, 37, 40 etc, but you can input whatever value you like. The calculator formula (A+2B)*25.4*C/D1, D2, Dn assumes that tyre-width equals the tyre's actual "height", i.e. half the difference between the outer tyre diameter and the rim seat diameter.
The next variable (C) is the chainwheel size.
Finally pick a nice set of freewheel sprocket sizes (D).
The result for each "speed" (yellow fields) is presented in the established "gear-inch" format. Best viewed with IE font size set to "smallest".