In the Dacia's gearbox design activities more often new steels with special mechanical properties (?m, HB, KCU) and acceptable thermal conductivity [4][6] are used. Because mechanical properties are determined by well-known and standardised methods and measurement means, in the subject of this study was decided the adoption of a faster method to determine the thermal conductivity of the new steel (40CD4X) towards the old steel (42CrMo4). The results of this experimental research confirm that to design calculation of the Dacia's gearbox, the values for the thermal conductivity from this study can be used.
