A tractor has hydraulically driven wheels at a cab end and castor wheels at an engine end. It can be driven cab forward in a working mode with a header on the forward end. It is rotated to engine forward in the transport position for more stable higher speed travel. The driven wheels are driven by hydraulic motors each having their own drive pump the output of which is controlled by a cam plate. The speed control is effected by a manually operable lever through a servo-cylinder which locates a speed control arm moving both cam plates to the set position. In a maximum speed position both cam plates are engaged against a stop. The steering is effected by moving the cam plates differentially to drive the wheels at different speeds. The piston rod of the servo-cylinder includes a spring relief so that the speed control arm can back off from the maximum position by moving the servo-cylinder body when steering in the maximum speed position.