A drive motor that may be an internal combustion engine or an electric motor is operated at substantially constant speed or, in the case of an internal combustion engine, at one of two substantially constant speeds. The speed of travel of the vehicle under various load conditions is varied by a drive control pedal operating a transducer that furnishes one input to a signal processor while a torque transducer in the engine drive shaft provides a second input to the processor. The processor output controls the positioning device that determines the transmission ratio of a continuously variable belt drive transmission by pressing together the halves of one pulley of the belt drive while separating the halves of the other pulley, varying their effective diameter with respect to the wedge-shaped belt in opposite directions. When an internal combustion engine is used as the drive motor, the engine speed can be selectably the maximum-torque speed or the maximum-power speed and the selection can be made automatic by means of limit switches operating at the respective extremes of the variable belt drive control range. When an electric motor is used as the drive motor, a compound-wound motor is operated at a speed in the transition range between behavior mainly as a series-wound motor and behavior mainly as a shunt-wound motor.