In this paper we discuss a method for determining heat transfer behaviour during tests in aerodynamic wind tunnels. The method involves computing the heat power evacuated by the heat exchanger as a function of the cooling airflow drag. Using dimensional analysis techniques, we define a Thermal Efficiency Coefficient (TEC). Systematic tests on three geometrically different types of vehicles show that there is a universal law governing the variation in TEC with cooling airflow drag. By associating the curve for this universal law with the characteristic curve for the heat exchanger alone, we can compute a TEC value for a measured cooling airflow drag value, and thus determine the vehicle's engine cooling performance during aerodynamic tests.