In thermodynamics and the theory of machines, the efficiency of a machine is the ratio of the useful work derived from a machine to the energy put into it. The mechanical efficiency of a machine is always less than 100%, some energy being lost as heat in friction. When the machine is a heat engine, the theoretical thermal efficiency can be found from the second law of thermodynamics but actual values are often rather lower. A typical gasoline engine may have a thermal efficiency of only 25%, a steam engine 10%. For very simple machines, efficiency can be defined as mechanical advantage divided by velocity ratio.