Exhaust heat recovery (EHR) system is attracting much attention as an effective means of improving fuel economy, especially at a low ambient temperature. Reducing the weight (heat mass) of EHR system is important for effective warm-up the engine at cold start, because recovered heat energy from exhaust gas is consumed to warm up the system itself. In this paper, we propose a new concept EHR system which applies a highly heat-conductive SiC honeycomb with thermal conductivity of 150W/mK, and also present vehicle testing results of two prototypes compared to the metal (stainless steel, 20W/mK) heat exchanger system on the market. SiC_prototype-1 adopted coaxial-bypass design with weight of 1kg, and steady-state heat recovery efficiency of 55% (at 36kg/h). SiC_prototype-2 is no-bypass (continuous heat recovery) type aiming for the minimum heat mass (0.4kg weight), and its heat recovery efficiency is designed to 30%, considering the acceptable cooling capacity of the vehicle at the highest engine load. The metal heat exchanger system with 4kg weight shows high efficiency of 65% in the steady-state. The vehicle tests (1.3L gasoline engine, WLTP driving cycle) revealed that each of the SiC_prototypes can improve fuel economy by 2.7%, in contrast to 0.9% improvement by the metal heat exchanger system. In summary, compact and light-weight (reduced heat mass) EHR system using the highly heat-conductive SiC honeycomb is promising to improve the fuel economy performance of the vehicle.