Electric, hybrid and plug-in hybrid drive trains are being considered as the future for road transportation. Private fleets such as taxis, buses, mail distribution or even state vehicles are usually firstly considered to introduce alternative technologies due to its smaller scale and routing knowledge. Usually no study of the optimal fleet conversion is made and only available alternative technologies, sometimes, oversized are considered. In this paper, a current conventional bus urban fleet is regarded to analyze the possibility to substitute the conventional vehicles by a more efficient fleet equipped with a battery and a hydrogen fuel cell. A methaheuristic method will be used with a vehicle simulation software to perform the optimal components selection of the hybrid electric vehicle (HEV) and plug-in hybrid electric vehicle (PHEV) powertrain. This model analyses an urban bus service requirements and regards the power and energy demand to the vehicle, to build a model able to do the optimal size and interaction of components, namely the tractive electric motor, electric battery, and fuel cell. The components are optimized with the objective to minimize cost and fuel consumption of the vehicle. Real measured driving cycles in Oporto city (Porto), Portugal, and the official European Transient driving Cycle, (ETC), are regarded to test the model. The results aim to give a more comprehensive knowledge for fleet conversion research/demonstration projects.