This work presents a system-level methodology developed to identify the optimum design of heat exchangers for Organic Rankine Cycle (ORC) Waste Heat Recovery Systems (WHRS) for automotive applications. The optimization of the evaporators is done following an iterative system-level approach, where system and vehicle outputs, such as the Fuel Economy (FE) and the System Payback Period are the objects of study. A 1D software has been developed to run an algorithm that, fed with corroborated assumptions, calculates the efficiency of the ORC cycle, the WHRS power output, the WHRS payback period, the FE potential and the Fuel Savings per year for different sets of evaporator designs. The algorithm identifies the optimum trade-off for evaporator efficiency, pressure drop, weight and cost to maximize the system Fuel Savings per year. The concept of the evaporator is a counter cross-flow heat exchanger; this is, the exhaust gas flows all along the outer case across the internal tubes. The working fluid flows within the tubes transversally in a meander path, in an overall counter current arrangement. There are several geometrical parameters open for optimization, such as the evaporator aspect ratio, cross section vs. length trade-off, arrangement of the tubes, shape of the outer case, number and diameter of the tubes, corrugation of the tubes, transversal and longitudinal corrugation pitch, etc. The resultant performance, size, weight and cost of the heat exchanger depend on which set of parameters is chosen. Moreover, every resultant heat exchanger output is linked in such a way that the optimum trade-off is not trivial.