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A test procedure has been developed to investigate the effect of lubricating oils on the mechanical efficiency of car manual transmissions. The test is based on the measurement of the power dissipated in a closed mechanical loop, in order to allow for better precision compared to traditional output power measurements. The test apparatus fits two equal transmissions, operated by an electric motor. For the present investigation, two Fiat 126 car transmissions were used and a two load stage test procedure was adopted. Lubricating oils having different rheological and compositional characteristics have been tested. Test results discussed in terms of the effects of operating parameters and oil characteristics on transmission efficiency indicate significant advantages for oils containing synthetic basestocks and friction modifiers. Correlation of the test with friction measurements performed with a conventional tribometer is also considered.