In this paper, a multi-mode parallel hybrid power-train so called clutchless geared smart transmission (CGST), is proposed and assessed in terms of the fuel economy. The use of a single motor/generator and a planetary gear enables the CGST to provide remarkable flexibility with four operational modes including the parallel hybrid mode. Based on its novel configuration, a fuel consumption model is newly developed and simulated to analyze the fuel economy of the CGST. The performance of the CGST is compared with those of the power-split hybrid system and the conventional manual transmission in city and highway cycles. The CGST shows the best efficiency in highway driving by reducing the unnecessary energy loss from the electric motor. Moreover, thanks to brake-regeneration and efficient engine operation with the specific operational modes, the fuel economy in the city cycle is comparable to that of the power-split hybrid system even though the CGST uses just a single electric motor.