Hi,
Internal Gear Ratio : 3.134
Gear Ratio using 20T : 1.8500 (37/20)
Gear Ratio using 19T : 1.9473 (37/19)
Gear Ratio using 18T : 2.0555 (37/18)
and so on.....
Final drive ratio using 20T = 3.134 X 1.8500 = 5.7979 (Rounding figure 5.80)
Final drive ratio using 19T = 3.134 X 1.9473 = 6.1028 (Rounding figure 6.10)
Final drive ratio using 18T = 3.134 X 2.0555 = 6.4419 (Rounding figure 6.44)
and so on.....
Hope it will help. Enjoy!