Due to ever-tightening CO2 regulations on passenger vehicles, it is necessary to find novel methods to improve powertrain system efficiency. These increases in efficiency should generally be cost effective so that the customer perceives that they add value. One approach for improving system efficiency has been the use of thermal energy management. For example, changing the flow of, or reusing “waste” heat from the powertrain to improve efficiency.Due to the interactions involved with thermal management, a system level approach is useful for exploring, selecting, and developing alternative solutions. It provides a structured approach to augment the right kind of synergies between subsystems and mitigate unintended consequences. However, one challenge with using these approaches early in a program is having appropriate metrics for assessing key aspects of the system behaviors. This paper discusses a metric for use in analytical studies of vehicle thermal management which enables comparison of system architectures and their implications on subsystem thermal interactions.A case study of underhood configurations is considered to illustrate the application of this metric. Inclusion and placement of underhood technologies such as active and passive aerodynamic systems (e.g., active grille shutters and belly pans, respectively) are evaluated. Using a system integrated thermal management model, such a metric enables decision-making regarding (1) technologies to be implemented based on benefits and trade-offs; (2) sizing and selection of components or entire systems for a chosen technology; and (3) the system level impacts of these decisions. This methodology yields system level thermal behaviors in a robust manner to provide comparative feedback about underhood architectures, with respect to some predetermined baseline.