An eight-degrees of freedom, two dimensional lumped-mass simulation model have been developed to study aggressivity of mass and stiffness ratio of vehicles in head-on collisions. Other factors like length of crumple zone, offset overlap and speed are also considered. The model assumes that the two colliding structures geometrically interact with each other. Engine assembly mass, primary and secondary stiffness parameters are considered in the model. Three injury risk criteria have been considered in this study; delta V or change in velocity of vehicle after impact, maximum acceleration sustained by the passenger compartment throughout impact, and length of deformation sustained by the car front. The most crucial aggressivity parameter is the mass ratio for the delta V criterion. The second and third aggressivity parameters are mass ratio and stiffness ratio for deformation length criterion. A slightly softer and longer crumple zone is proposed to be used for heavier vehicles to achieve compatibility of crashes of vehicles of different mass. This measure would alleviate aggressivity by a factor of about 20%. It was found that both mass and stiffness ratio have no aggressivity on the partner vehicle, as far as acceleration injury criterion is concerned. Acceleration is determined by the vehicle’s own mass and stiffness.