@ GiraffeEars- You are asking a question that relates to the laws of thermodynamics. While the law of conservation of energy states that all energy must be accounted for, the second law of thermodynamics attests to the chaotic nature of energy. The potential energy transferred from one source to the next will always be less than the original source. Energy can change, but cannot return to its original state in whole. The chemical energy that from the gasoline not only becomes mechanical energy, it becomes heat, and work on other components as friction, and potentially static electricity. The efficiency of an engine depends on the amount of energy transferred from a given amount of chemical potential energy to mechanical kinetic energy.