Internal combustion engines are actually one of the most important source of pollutants and greenhouse gases emissions. In particular, on-the-road transportation sector has taken the environmental challenge of reducing greenhouse gases emissions and worldwide governments set up regulations in order to limit them and fuel consumption from vehicles. Among the several technologies under development, an ORC unit bottomed exhaust gas seems to be very promising, but it still has several complications when it is applied on board of a vehicle (weight, encumbrances, backpressure effect on the engine, safety, reliability). In this paper, a comprehensive mathematical model of an ORC unit bottomed a heavy duty engine, used for commercial vehicle, has been developed. The model is completed with the sizing of the two exchangers involved in the ORC plant: the heat recovery vapor generator (HRVG), which affects the backpressure at the engine exhaust, and the condenser, which is realized with the similar technology of a radiator, and it affects vehicle frontal area occupation. Also weights have been calculated. Thanks to the model, a wide analysis of the recoverable power and the exchanger sizing has been performed on the ESC-13 engine working points. For each engine working point the backpressure effect due to the presence of the HRVG has been evaluated and overall ORC unit weight has been estimated. The extra propulsive power related to weight increase has been estimated considering the ETC cycle (which is the correspondent transient cycle of the ESC-13 modes) and it is compared to the ORC mechanical power recovered and the overconsumption due the backpressure effect. A real net power recovered is, therefore, assessed.