The design of an efficiency optimal controller for the variator in a vehicular continuously variable transmission is studied. A conventional controller aims at tracking a prescribed reference for the transmission ratio and at preventing damage, but does not address efficiency. Sufficiently accurate models for the efficiency as function of the clamping forces are not available, whereas measurement of the efficiency requires extra sensors. In this brief, a controller is proposed that improves the efficiency without needing extra sensors. The maps between the clamping forces (input) and the efficiency or the speed ratio (output) are studied with test rig experiments. These maps exhibit a maximum, but the location of this maximum is uncertain. So, an extremum seeking controller is developed. This controller can adapt the input to maximize the output, without needing a model. Experiments show that this approach is feasible and that a conventional controller is outperformed. A robustness analysis for disturbances indicates that these are effectively handled.