Nanocoolants are engineered colloidal suspensions of nanoparticles in a conventional coolant (water–ethylene glycol). Use of metals and metallic oxide nanofluids at concentrations greater than 0.01 vol % has been studied in greater detail compared to functionalized graphene nanofluids. In the present work, an experimental investigation is carried out on an automobile radiator by changing the conventional coolant to reduced graphene oxide (RGO) nanocoolant. The present study focuses on the effect of ultralow nanoparticle concentration (0.002–0.006 vol %), Reynolds number of hot coolant (80–170), Reynolds number of air (210–270), and inlet temperature of hot coolant (40 °C and 60 °C). The effect of these parameters on the performance of the radiator is measured in terms of convective heat transfer coefficient (CHTC), Nusselt number, friction factor, and effectiveness. Results indicate that at 0.006 vol % concentration, there is a maximum increase of 100.5% and 111%, respectively, in CHTC and effectiveness, besides a reduction in friction factor. RGO nanocoolant offers great scope for reducing the radiator size and increasing its performance.