For transportation and defense systems, equations of state describe the thermodynamical behavior of gases, as a function of pressure, density, and temperature and are often used to close systems of equation (such as Navier–Stokes for fluid mechanics). For low pressure system (under a few dozen MPa), the ideal gas equation can be used, whereas for high pressure system (above 1000 MPa), dedicated equations of state are used (Jones-Wilkins-Lee: JWL or Becker-Kistiakowsky-Wilson: BKW). For intermediate pressure systems, such as interior ballistics or pyromechanisms, where the pressure is around a few hundreds MPa and many phenomena interact closely and simultaneously (multiphysics system: combustion, fluid dynamics, thermics, fluid-structure interaction, thermodynamics…), the ideal gas equation is not valid anymore and nor are the JWL or BKW equations. A dedicated equation of state is then necessary for intermediate level of pressures, and its choice is not obvious. This paper presents a comparison of equations of state for this range of pressure (Noble-Abel, Van der Waals, virial), details the calculations of the involved coefficients and their effects on uncertainties, and studies the influence of the equation of state on the predictions of pressure, density, pressure coefficient, and heat transfer, with both analytical and numerical approach. The results highlight the importance of the choice of the equation of state, especially for a multiphysics system, as the equation of state influences not only thermodynamical properties (pressure, density, temperature) but also thermical properties of a system, such as heat transfer.