The algorithm used in the third version of the Calspan Reconstruction of Accident Speeds on the Highway (CRASH3) and planar impact mechanics are both used to calculate energy loss and velocity changes of vehicle collisions. They (intentionally) solve the vehicle collision problem using completely different approaches, however, they should produce comparable results. One of the differences is that CRASH3 uses a correction factor for estimating the collision energy loss due to tangential effects whereas planar impact mechanics uses a common velocity condition in the tangential direction. In this paper, a comparison is made between how CRASH3 computes the energy loss of a collision and how this same energy loss is determined by planar impact mechanics. The main factors that control energy loss as calculated by CRASH3 are the determination of the PDOF (principal direction of force), definition of a common impact point of the two vehicles, the common normal velocity condition and the tangential correction factor. In the planar impact mechanics solution, the controlling factors are the definition of a crush surface, definition of a common impact point, common velocity conditions and the values of normal and tangential coefficients. Experimental collisions (RICSAC, Research Input for Computer Simulation of Automobile Collisions) are used to provide a basis for comparison. A method is proposed that exploits the features of both methods for vehicle accident reconstructions.