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This paper introduces mathematical equations for power loss calculations of major automatic transmission components. It also presents a theoretical investigation on contributions of each component's power loss to transmission total power losses. A computer program for automatic transmission efficiency calculation has been developed based on mathematical equations. Efficiency calculations for the Hyundai A4BF1 electronic 4 speed automatic transmission has been executed in order to validate the program. The calculated efficiency under various input and operating conditions - such as different input torque & RPM, transmission gear state, ATF temperature, torque converter clutch on/off - showed quite good agreement with the test results.