Among the most pressing needs in the development of rational approaches for vehicle handling quality evaluation is a method for relating the driver's actions with the car's performance capability. This paper describes a concept which provides a graphic and objective measurement of this relationship, including the effect of road surface conditions on overall system behavior. Based on a continuous plot of the vector acceleration of the vehicle in the horizontal plane, the resultant figure, which is called a g-g diagram, is shown to be useful for characterizing the vehicles's envelope of performance, the variation of this envelope with tire-road surface conditions, the accessibility of various operating points to the driver in performing a task, performance margins utilized by the driver in normal operation, and for several other applications. Experimental results from both proving ground and on-road tests are analyzed in terms of driver willingness limits, driver idiosyncracies, and task descriptions. It is shown that drivers usually employ healthy margins of performance on dry roads and that the normal modes of operation are predominantly along the axes (pure cornering, pure braking, and pure driving). Applications of the method to handling studies, driver training, and general vehicle research and development are briefly described.