This thesis work, conducted with the direct collaboration of Iveco Group, aims to evaluate the performances, intended as power, energy and heat rejection capability, of two different configurations of an Heavy Duty Fuel Cell Electric Vehicle (FCEV). The company provides the vehicle's parameters and its real component data, along with the system's longitudinal dynamic simulations over two routes that are regularly traveled by vehicles of the same size and category. These simulations give comparative results for the two configurations, which differs by the implemented fuel cell system, one having an output power availability greater than the other. The resulting data is analyzed and elaborated to evaluate the performance of the two FC systems, that affect the overall vehicle's operation.