Theoretical consideration of the energy absorption properties of structures shows that the specific energy absorption (E/Mk) is related to the normalised crush depth (d/L) and to a stress/density parameter. Examination of the crush characteristics of the car population shows that the mean stress/density parameter of the car population in frontal crush is independent of car size. On this basis regression is carried out which relates (2×E/Mk)½ to normalised crush depth (d/L)⅔Using data from nine staged frontal car to car collisions this generalised specific energy absorption representation is compared with CRASH 3 and with Prasad's (5) individual car reformulation. It is shown that the accuracy of Delta V prediction with Prasad's individual car reformulation is superior to both the normalised crush depth model proposed here and to CRASH 3, both of which have similar accuracies. A missing vehicle energy estimator, E1×[1+Mk1/Mk2] is examined as is the crush depth energy estimator, E1×[1+d2̄/d1̄]. lt is shown that in conjunction with the normalised frontal crush depth regression they can be applied to frontal and to 90 degree front/side car to car collisions.