I. ABSTRACTTwo commercial vehicle power steering systems that provide speed proportional variable efforts and reduced power consumption were developed and tested on a typical Class 7-8 line-haul tractor. Both systems, designated by TRW as the X-4 V-Tronic series, are a variable-flow power assist concept. The first system is the X-4 VTE electronically controlled system with driver selectable switched “NORMAL” and “ECONOMY” modes. The second system is hydraulically controlled, designated as the X-4 VTLS, and uses steering load sensing. Providing increased steering road feel at highway speeds and reduced parking efforts, both systems enhanced an optimized current production conventional fixed flow-control system. Test results show that the system's power consumption was also significantly reduced.