Not every tractor has a continuously variable transmission.
And even the continuously variable transmissions are not as simple in construction as the standard one in farming simulator.
This script adds on the basis of the file gearboxAddonConfig.xml a gearbox to all configured tractors.
Many various transmission kinds can be simulated. There are classic gearboxes with gears, one or two groups and reverse gears or groups.
You can select five gearbox: automatic, manual.
It is also possible downshift

Credits:
Mogli aka Biedens