Gas internal combustion engines are typically 18-23% efficient in terms of mechanical energy out vs chemical potential energy in. High performance and forced induction engines can to a little better than that but not by much. You lose a lot of energy through heat, sound, vibration amongst other things. Also emissions really damp efficiency but are obviously important. It would be cool to see a car manufacturer build the most efficient, light and not overpowered car possible to see just how much they could stretch the mileage.