To solve the problems of conventional devices for speed control such as an automatic gear change, toroidal CVT, etc., for an automobile, for example, that the automatic gear change has large weight and gear friction, and gives a shock at the time of acceleration when the gears are decreased in number, and that the toroidal CVT is unusable for large vehicles due to its mechanical structure.

For the third speed control system, hydraulic pressure is used, and a pump and a motor are combined in a set, in such a way that the capacity of the pump is varied to adjust the rotation of the motor. The pump and the motor are shaped semispherical, and covered with a spherical cover to rotate around a wheel track as a rotating pump. By thus varying the contact angle to the hemispherical faces, the capacity of the pump can be adjusted.