Accident Investigators have occasionally and mistakenly assumed that the magnitude of the Change of Speed (δV) during an auto collision is simply proportional to the post collision crush of the automobile. This paper reviews the introduction of the Vehicle Deformation Index, Collision Deformation Classification, Barrier Equivalent Velocity (BEV), Stiffness of the other vehicle, and the appropriate δV. Presented are some practical worked examples deriving δV and comparing it with crush and BEV for each deformed automobile. Change of Speed is also reviewed for crashes into deformable barriers and compared with post collision deformation and BEV. The crush magnitude of three types of non-symmetrical frontal collisions are studied and compared with BEV. The crash test stiffness of recent models and the CRASH 3 crush parameters are compared.