A significant driver for the development of future commercial vehicles is likely to be the introduction of fuel consumption related legislation in various regions around the world. The application of a waste heat recovery system to the powertrain of such vehicles is seen as a possible step, amongst many, to help them achieve the required fuel economy. In particular, the Rankine Cycle (a closed steam cycle) is often proposed as a potential means for deriving work from the engine exhaust heat.Rankine Cycle systems are already in use in off-highway applications, such as stationary engines or marine power-packs. However, the technical and commercial viability of these systems for on-highway, principally long haul truck application is as yet unproven. Aspects such as the in-use economy benefits, the system performance density, the component robustness and all interactions with the other vehicle systems have to be evaluated. These aspects translate into new challenges for suppliers of vehicle thermal management components and modules.In this paper, the simulation the Rankine Cycle system together with the complete vehicle infrastructure using in-house developed tools, in order to predict the actual likely fuel consumption benefit, is presented. Results are shown in relation to the use of alternative system layouts for a long haul vehicle application. The validation of the simulation results is also presented.