This paper proposes a numerical scheme for the fast calculation of an efficiency map in Permanent Magnet Synchronous Motors (PMSMs). The efficiency map is the main performance feature required to design traction motors for electric vehicles. To obtain the efficiency map, iron losses must be calculated across wide speed and current ranges. The iron losses can be estimated from magnetic flux waveforms, which require a huge amount of non-linear time-stepping finite element analysis (FEA) solutions. In this work, the flux waveform estimation scheme is proposed aiming to reduce the number of required FEA solutions. The proposed scheme removes the redundant FEA calculation based on the fact that the time variation of the field at one position is matched with the space variation of the field at one time step in PMSMs. To verify the effectiveness of the proposed scheme, not only the flux waveform, but also the iron losses and efficiency map of a PMSM is calculated using both conventional and proposed approaches. The comparison results show that the computation time to build a single efficiency map can be reduced from 8.8 hour down to 1.1 hour within around 5% errors.