This article is devoted to the production and testing of the performance of electromagnetic fuel saver. Purpose of the work is to analysis effect of varying of core of electromagnetic fuel saver to the performance of the internal combustion diesel engine. Materials for core of electromagnetic fuel saver are made of plain carbon steel and copper. Diameters of the wire winding, which is used in the research, are 0.25 mm and 0.35 mm. Speed of the engine, and number of coil which is coiled in the winding core of the fuel saver are chosen as the testing variables. The produced fuel saver is tested in the laboratory and on the road in the traffic jam condition and in the highway. Measured variables in the laboratory is the specific fuel consumption and measured variable for road testing is fuel consumption. From this work is obtained that the performance of the electromagnetic fuel saver which use copper core is better than the electromagnetic fuel saver which use steel core and permanent magnet.