Derailleurs + mountain-drive (m-d), speed-drive (s-d), high-speed-drive (h-s-d)
Some typical tyre dimensions: Circumference in meter
  Brompton (Standard) 1.32
Birdy 18" (Marathon Race) 1.36
  20" 40-406 1.53
20" 47-406 1.55
  20" 54-406 1.59
24" 47-507 1.86
  24" 54-507 1.92
26" 40-559 2.00
  26" 57-559 2.10
28" 25-622 2.13
  28" 37-622 2.19
28" 40-622 2.22
  28" 50-622 2.28
28" 60-622 2.35
 
Input:
Circumference of rear wheel (in meter)
Tooth count of front chainring
Rear cog
   
   
gear-inches
Output:      1 : 1      m-d      s-d      h-s-d
1. speed
2. speed
3. speed
4. speed
5. speed
6. speed
7. speed
8. speed
9. speed