Your vehicle's gearbox multiplies torque. The engine's cylinders produces strokes with each turn, and this rotates the crankshaft's flywheel. The flywheel turns with only a fraction of the torque necessary to drive the vehicle. The gearbox, better known as the transmission, raises this torque by meshing together differently sized gears. Driveshafts then transmit this torque to the wheels' rotors, turning them and forcing the vehicle forward.
Multiply the force acting on the box's first gear, in Newtons, by the gear's radius, in meters. If, for instance, 3000 Newtons act on a gear with a 0.3 meter radius: 3000 * 0.3 = 900 Newton meters.
Divide the first gear's radius by the second gear's radius. If the second gear has a radius of 0.15 meters: 0.3 / 0.15 = 2. The gearbox's torque ratio is 2:1, and its torque multiplication factor is 2.
Multiply the answer to step 1 by the answer to step 2: 900 * 2 = 1,800 Newton meters. This is the gearbox's output torque.
If you know your gear ratio, use it in step 2 rather than calculating it.