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Another factor that people sometimes compute for machines is their efficiency, or the ratio of the work that results to the amount of work put into the machine. The efficiency of a machine is usually expressed as a percentage and can vary from 5 percent to 95 percent. A perfect machine would be 100 percent efficient. Most simple machines are very efficient, but they always lose some efficiency due to friction. An automobile engine is much less efficient because much of the energy used to move the crankshaft is lost to friction in the form of heat dissipating from the engine.