A power assisted rack and pinion steering gear is mathematically modelled, enabling the development of quantitative definitions for power steering ‘stiffness’ and ‘feel’. Both these quantities are defined as instantaneous derivatives, being a function of various valve parameters such as differential boost rate, effective pinion pitch radius, piston area, torsion bar stiffness, etc. The resulting stiffness and feel relations are plotted for two types of valve boost curve: a conventional tan-function-like boost curve and a linear boost curve. The latter is shown to yield proportional feel and stiffness in the cornering region of valve operation. The application of this theory to the performance evaluation of two families of speed sensitive valve systems is also addressed.