Modern heavy duty diesel engines can reach brake thermal efficiencies of up to 50% by utilizing waste heat recovery (WHR) technologies. The effect of ORC WHR on engine brake specific fuel consumption is a compromise between the fuel penalty due to the higher exhaust backpressure and the available power from the WHR system that is not attributed to fuel consumption. This work focuses on the fuel efficiency benefits of installing an ORC WHR system on a heavy duty diesel engine. A six cylinder, 7.25 heavy duty diesel engine is employed to experimentally explore the effect of backpressure on fuel consumption. A zero-dimensional, detailed physical ORC model is utilized to predict ORC performance under design and off-design conditions. The ORC model includes a detailed exhaust gas heat exchanger model and a thermodynamic ORC submodel to explore the effect of recovering various amounts of waste heat on ORC system efficiency under the same engine load and speed conditions. This study focuses on maximum engine power conditions where the exhaust gas and temperature are maximized. As expected, the results show that increasing the heat exchanger surface area leads to higher heat recovered at the expense of higher exhaust backpressure and WHR system weight increase. Nonetheless, the heat recovery returns from increasing the heat exchanger surface area are increasingly diminished at the expense of backpressure as the between the fluids approaches zero. At the same time, the weight increase of the heat exchange is illustrated as the main parameter that limits the ORC system design. The optimum heat exchanger length is a trade-off between exhaust backpressure, the required net ORC power and the weight increase.