Compatibility between cars is a major issue worldwide. In Europe several groups are dealing with it: BRITE EURAM, which involves the majority of European manufacturers, and EEVC WG15, involving several European test laboratories and institutes, are the most active in Europe at the moment. In the past also ADAC developed a similar study in Germany.In US NHTSA is conducting studies, mainly related to the particular American market, where pick-ups represent the most important cause of incompatibility.Several approaches and test procedures are proposed to assess compatibility which consider tests against fixed obstacles, like offset deformable barriers and full width barrier (to measure energy absorption and stiffness of the front end and its distribution) and overload tests (to measure resistance capability of the compartment). All these kinds of tests are aimed mainly to measure structural behaviour of the cars, without taking into account the actual deceleration pulse that a car sustains in a real accident.US are considering the use of a mobile defomable barrier, which gives, as advantage, the possibility of reproducing the actual deceleration pulse of the vehicle. The problem associated with it is the complexity and the repeatability of the test, which suggests to investigate more common test configurations more deeply.Within EEVC WG15 activities one of the authors explored the compatibility between cars in frontal collision, starting from conservation equations of classical mechanics. A “compatibility equation” was suggested to describe how vehicles share energy during crash.In this paper the described approach has been further developed theoretically, in order to take into account not only the structural behaviour of the car (particularly the front end) but also (and mainly) the deceleration that occupants sustain during crash.. The result is a proposal for a new crash test procedure: this involves a new fixed deformable barrier concept, which is able to reproduce the structural and biomechanical behaviour of a compatibility reference car-to-car impactBy means of the same test, the self protection level for the tested car is guaranteed.