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The article presents the results of tests on a Fiat 0.9 TwinAir engine fueled with unleaded gasoline with an octane number of 95 and gaseous fuels LPG and CNG. The purpose of the research was to determine and compare the obtained efficiency indicators of the engine fueled with the tested fuels. The article presents the following efficiency indices: indexed efficiency ηi, mechanical efficiency ηm and total efficiency ηo. The afore mentioned indices were determined during engine operation under steady-state conditions. In addition, the methodology for determining the indices evaluating the efficiency of the engine is presented. During the tests, the engine was operated according to load characteristics with crankshaft speeds of 1900, 2800, 3700, 4600 and 5500 rpm, respectively. The engine tests were conducted on an engine dynamometer, allowing measurement of crankshaft speed, torque developed by the engine and fuel consumption. The results of the tests showed differences in the efficiency of the engine when it was fueled with different fuels, which indicates the varying degree of conversion of the energy contained in the fuels into mechanical work. The Fiat 0.9 TwinAir engine was characterized by very high mechanical efficiency reaching values of more than 90% over a wide speed range. The high mechanical efficiency could have been achieved by downsizing. The mechanical efficiency values obtained may also have been favorably influenced by the use of MultiAir technology, which eliminated the number of kinematic pairs that could cause additional mechanical losses and increase the resistance to motion of the valve train. The engine's overall efficiency reached an average value of about 25%. The engine achieved the highest overall efficiency values in the medium load range. As the load increased, the engine's efficiency decreased. The research helped reveal the most favorable speed-load conditions of its operation. They demonstrated the favorable effect of LPG fueling on the engine's overall efficiency.