As the number of vehicles equipped with power steering systems increases, the need for speed proportional power steering systems has been increasing. They can provide light steering effort at low vehicle speed and high steering effort at high vehicle speed. Therefore, a speed proportional steering system is a better steering system for the driver.A newly developed rotary valve for speed proportional power steering systems changes its hydraulic valve gain characteristics. The operating principle is completely different than those of conventional flow and hydraulic reaction control systems.The rotary valve developed has 2 sets of orifices with different characteristics. They are arranged in series to achieve variable power assist by changing the bypass ratio between those orifices using a solenoid valve.This system offers an excellent degree of modulation between high speed mode (minimum power assistance) and parking mode (maximum power assistance). It is, moreover, simple, compact and economical.