When designing the vehicle cooling system, accurate knowledge of the required airflow through the heat exchangers is necessary for proper specification of the cooling fan, the heat exchangers, and the associated electrical loads. The simplest method of expressing the engine cooling fan performance requirement is based on the “open air” performance curve measured on the airflow test chamber, excluding effects of the heat exchangers and vehicle environment. However, the difference between open air and on-system airflow performance and noise (installed on the heat exchangers) can be significant due to the influence of the heat exchangers, fan shroud, and downstream blockage on the airflow through the fan. If these factors are neglected in the evaluation of the cooling fan, incorrect specification of the fan performance can result.In this paper, two case studies are presented in which different fan modules, designed for the same vehicle applications, are compared both in open air and installed on the heat exchangers. In both cases, open air test results lead to erroneous conclusions concerning power consumption and noise for a given on-system airflow rate. Misleading comparison results such as these could lead to incorrect selection or specification of the cooling fan, resulting in inadequate cooling, excessive power consumption, or excessive noise. Accordingly, it is recommended that cooling system designers specify cooling fan performance using on-system test conditions. A list of factors which should be considered in specifying on-system performance is presented.