To meet the increasing need of assistant software tools during the design of new electronic architectures in passenger cars, we developed a simulation tool as part of a project supported by the grant No 033 of the French National Research program PREDIT 2 (1998-2000). This paper describes a modular approach for system modelling and the related simulation tool, showing its usefulness for the architecture performance evaluation and validation. Based on reusable component models (µC, Schedulers, CAN, VAN, etc.), a simulation software core (SES Workbench or Opnet) and a friendly interface, an architecture can be easily built and then simulated. We used Carosse-Perf to model and simulate a multiple network embedded system. Thanks to the modularity of models, we easily tried several configurations (task priorities, task distribution) and we revealed that a fare load balancing does not necessarily lead to optimal solution in terms of task response times.