Alternative vehicle powertrains (hybrid, hydrogen, electric) are among the most interesting solutions for environmental problems afflicting urban areas. Electric and hybrid vehicles are now slowly taking place in the automotive sector, but on a Tank To Wheels (TTW) basis, the most effective alternative powertrain is surely represented by Fuel Cell Electric Vehicles (FCEV): those fuelled by hydrogen seem to be the ones closest to market. The design of a FCEV however, is not straightforward and involves several issues (fuel cell sizing, hydrogen storage, components efficiency, sizes and weights). Basing on these considerations, the Authors present a software procedure for the optimal design of the components of a passenger FCHEV (Fuel Cell Hybrid Electric Vehicle). A comprehensive energy balance of the whole vehicle during a driving cycle has been implemented in order to find the overall optimal sizing and control strategy of the fuel cell, the energy storage system (ESS) and the hydrogen storage system. The propulsive power needed to run a car on a given reference driving cycle, in fact, may be given by the two on-board power sources: hydrogen and electricity, stored in proper ESS. At the same time, power requirements depends also on the whole vehicle weight, which comprehends fuel cell, batteries and fuel tank weight and hydrogen amount (each of which having to be opportunely evaluated in relation to designing parameters). In particular, fuel cell and battery power have to fulfill the traction power request, while fuel tank and hydrogen amount (which may be stored on-board through various different technologies and at different thermodynamic conditions: gaseous or liquefied at different temperatures and pressures) have to fulfill vehicle mileage requirements. Different designing options of electricity and hydrogen on-board storage technologies are here compared by the Authors, in order to evaluate the effect of various design parameters (including mileage, FC maximum power output, hydrogen storage pressure and others) on the overall performances of the vehicle (including its weight and overall energy consumption).