Real-time extensions to standard switched Ethernet widen the realm of computer networking into the time-critical domain. These technologies have started to establish in process automation, while Ethernet-based communication infrastructures in vehicles are novel and challenged by particularly hard real-time constraints. Simulation tools are of vital importance to explore the technical feasibility and facilitate the distributed process of vehicle infrastructure design. This paper introduces an extension of the OMNeT++ INET framework for simulating real-time Ethernet with high temporal accuracy. Our module implements the TTEthernet protocol, a real-time extension to standard Ethernet that is proposed for standardisation. We present the major implementation aspects of the simulation model and apply our tool to an abstract in-vehicle backbone. A careful evaluation that compares our results with calculations obtained from a mathematical framework, as well as with real-world measurements using TTEthernet hardware shows simulation and reality in good agreement.