To control transmission ratio of a continuously variable hydrostatic transmission including an input shaft (14), an output shaft (16), a hydraulic pump unit (18) driven by the input shaft (14), a grounded hydraulic motor unit (20), and a wedge-shaped swashplate (22) drivingly, pivotally connected to the output shaft (16) and positioned to accommodate pumped hydraulic fluid exchanges between the pump and motor units (18, 20). A ratio controller is provided having a pair of hydraulically actuated, differentially sized pistons (58, 68) coupled by a linkage mechanism to pivot the swashplate (22) in opposite transmission ratio-changing directions. The smaller piston (58) is incorporated internally of the output shaft (16), while the larger piston (68), of an annular shape, coaxially surrounds the output shaft (16). The linkage mechanism has a geometry effective to translate axial forces exerted by the pistons (58, 68) into amplified ratio-changing moments exerted on the swashplate (22). The ratio controller also includes a control valve (78) selectively positioned to change the hydraulic fluid pressure acting on the larger piston (68), such as to set a transmission ratio and also to determine the ratio-changing direction of swashplate pivotal motion.