A system level, steady-state thermodynamics model for an automotive PEM fuel cell system was developed to investigate the effects of vehicle speed and operating pressure on the size of the system components, heat and water formation, fuel consumption, fuel cell, and system efficiency. Moreover, the consequences of the choices of membrane's thickness and ionic conductivity on thermal and system efficiencies as a function of vehicle speed are analyzed. The model consists of a PEM fuel cell stack model and supplementary models for all necessary auxiliary components. Results show that increasing the system pressure has a significant positive impact on the water balance characteristics of the system, since less water is needed to reach the required relative humidity levels for the reactants. On the other hand, the high-pressure system requires more power in order to operate the compressor, which in turn, decreases the system efficiency. Decreasing air stoichiometry at high operating pressures improves the water management but has no effect at low pressures. Results also show that increasing the membrane's ionic conductivity and decreasing its thickness enhances the system efficiency especially at higher vehicle speeds. Lastly, it is obtained that the system efficiency decreases as the vehicle speed increases.