We can use a common definition which says that when a force is applied to a mechanical system, it will typically reach steady state after going through a few transient behaviors. The difference in performance between a stabilized and a transient mechanical system may be substantial. Loss of power during vehicle acceleration may be determined with a roller dynamometer. In this paper we determine the power loss in the car transmission as well as the power loss taking rolling resistance into consideration, in the transient acceleration mode, up to the maximum speed of the vehicle. We also highlight the speed performance values in comparison with the values given by the manufacturer with a view to ensuring the accuracy of transmission loss results.