Water/ethylene glycol (W/EG) mixture is a common heat transfer fluid in vehicle radiators that exhibits poor thermal performance. It can be substituted by nanocoolants (nanofluids as coolant) to enhance the overall heat transfer performance of radiators. However, addition of nanoparticles to enhance heat transport may be accompanied by a simultaneous increase in pumping power. In the present study, an experimental evaluation of thermohydraulic performance of graphite nanocoolant (W/EG-based graphite nanofluid as coolant) in vehicle radiators is carried out by utilizing an in-house test rig. The thermal performance of the nanocoolant and the base fluid is compared at the same Reynolds numbers, coolant mass flow rates, and pumping power. The overall heat transfer coefficient is augmented with the use of a nanocoolant, while comparing with the same Reynolds number and coolant flow rate criteria. The enhancement is higher at lower air and coolant mass flow rates and gradually decreases as the flow rates increases. The same pumping power comparisons demonstrate that for low pumping power cases the overall heat transfer coefficient of the nanocoolant is higher than the base fluid, and the trend converses as the pumping power increases to higher values. The performance index, which indicates the net enhancement or diminution of thermal performance relative to the pumping power, is relatively more for graphite nanocoolant at lower coolant and air mass flow rates but diminishes in experiments with higher flow rates. This study shows that an analysis for finding the sweet spot is essential before applying graphite nanocoolant in vehicle radiators. © 2019 by Begell House, Inc.