First gear:

The gears in the gear box are illustrated in Figure below:

                       Conventional Gear Box

 For providing first g ratio, gear A meshes along gear B and gear H meshes along gear G.

For smallest size of gear box = t_B/t _A≈ t_G /t_H

If          t_A = 20 teeth    ∴          t_H = 20

∴          t_B = 2 × 20 = 40 teeth  and      t_G = 20 × 2 = 40 teeth

As centre distance must be same

∴          t _A  + t_B  = t_C+ t_D  = t_E  + t_F = t_H + t_G

               t_C  + t_D  = 40 + 20 = 60    . . . (10.1)

                   t_E  + t_F = 60                     . . . (10.2)