This paper presents a case study in computer-assisted tolerancing with the configuration space method. We analyze part of a gear selector mechanism in an automotive automatic transmission. The model contains three complex parts with 99 functional parameters. The analysis, which takes less than a minute on a workstation, indicates that the critical kinematic variation occurs in third gear and identifies the parameters that cause the variation. The analysis program handles general planar systems of curved parts with contact changes, including open and closed kinematic chains. It computes the worst-case variation in the system kinematic function and detects potential failure modes due to unexpected qualitative variations in function, such as jamming. It constructs configuration space models of the kinematic variations of the pairs in the system then derives the system variation by composing the pair models.