In the last two decades the front end structures of all cars were optimized in crashes against rigid barriers in order to guarantee the highest level of self protection. As a consequence, the stiffness of the front end structure was directly related to the own mass of the car. This effect is independant of the crash test speed if safety standards demand the same test speed for all cars. Although German accident statistics still indicate that twice as many fatalities of car occupants occur in single vehicle accidents as in car-to-car collisions, the problem of compatibility has to be taken into account because of the high number of injuries in car-to-car collisions. Therefore, for compatibility reasons, for years experts have recommended higher crash speeds for small cars and lower crash speeds for large cars; a demand which of course is not in accordance with the safety standards. Based on the state-of-the-art cars, experts agree that the stiffness of the front end of a small car should be increased and the stiffness of the front end of a large car should be decreased. This energy-balance concept is briefly described in this paper.