A fixed differential gear system. This arrangement has a typical ratio of between 15:1 and 100:1. The ratio is given by:

${\displaystyle m_{G}={\frac {1}{{\frac {1}{\left({\frac {N_{R2}}{N_{S2}}}+1\right)}}-{\frac {1}{{\frac {N_{R1}}{N_{S1}}}\left({\frac {N_{S2}}{N_{R2}}}+1\right)}}}}}$

where:

${\displaystyle N_{S1}}$ is the number of teeth on the No. 1 Sun gear (S1, red, on the left)

${\displaystyle N_{S1}}$ is the number of teeth on the No. 2 Sun gear (S1, red, on the right)

${\displaystyle N_{R1}}$ is the number of teeth on the No. 1 Ring gear (R1, orange, on the left)

${\displaystyle N_{R2}}$ is the number of teeth on the No. 2 Ring gear (R2, orange, on the rightn)

${\displaystyle N_{P1}}$ is the number of teeth on the No. 1 Planet gear (P1, dark blue). These gears rotate about fixed axes.

${\displaystyle N_{S2}}$ is the number of teeth on the No. 2 Planet gear (P2, grey)