Abstract: A differential gear and control system for motor vehicles having a single driven axle and at least one undriven axle comprises a differential gear and a differential lock, which is adapted to be controlled in dependence on the speed of travel of the vehicle. In order to increase the traction when the vehicle is started whereas the handling of the vehicle should not adversely be affected by a locking of the differential gear, the differential lock is adapted to be controlled so that it exerts on the differential gear a restraining torque which decreases as the speed of travel of the vehicle increases from zero to a predetermined speed of travel.

Abstract: In order to provide in a liquid-containing liquid friction coupling an air-filled space having a predetermined volume in a process which requires a minimum of manipulation and can be carried out in a short time, equal quantities of air are forced into the liquid-containing liquid friction coupling, on the one hand, and into a pressure-tight reference vessel which contains an air-filled space having the desired volume, on the other hand, the pressure difference between the reference vessel and the liquid friction coupling is then measured, and the pressures are balanced in that liquid is added to or removed from the liquid friction coupling.