In parallel to and with the development of numerical methods for calculating flow around road vehicles, experimental techniques are improving in order to obtain physical information at the same level of precision as those given by computation. In this way, RENAULT has developed in 1984 a numerical tomography technique which permits an analysis of vortical flow in wakes of road vehicles. Nevertheless, interpretations of the maps obtained by this method are sometimes difficult for complex aerodynamic shapes, mainly concerning the source of free share layers. By coupling this last method with oil-flow visualizations, and a simultaneous reading, it is possible to overcome this problem and to obtain a more complete understanding of flow structures around a car. Results of this technique are presented for some typical models, and discussed.