Up to now, incompatibility of cars was mainly derived from theoretical considerations due to differences with regard to mass, stiffness, and geometry. Numerous accident analyses and reports only showed, that mass is a dominant factor for the injury outcome in car-to-car collisions. The same analyses have difficulties to assess the influence of stiffness and geometry. One reason is the wide scattering of the injury outcome due to other more important variables than incompatibility. A second reason is the fact that stiffness correlates more or less with the mass. A third reason is the effect of the non-linearity of the geometry influence that means a positive influence of incompatible weak structures causes a reduced acceleration, but with an increased risk of intrusion into the passenger compartment. This can be observed at high impact speeds and/or low overlap degrees. These effects can be seen from accident analysis. Nevertheless, on the basis of theoretical considerations, the aspect of compatibility can be implemented in the design of new cars. This is true for a certain range of impact severity and for a limited number of car size groups only. A realistic approach in this regard is detailed in the paper.