More specifically, a typical internal combustion engine can turn about 20% of the available chemical potential energy into useful mechanical energy. The possible efficiency of a fuel cell car is somewhere between 40 and 60%. Even if it is the low-end, that's still twice the useful work, or half the necessary fuel, when compared to gas engines.
When other energy recovery devices are included, such as Peltier junctions or other thermo-electric gizmos, and current hybrid style brake-to-electric-generator technology, the fuel cell car's efficiency is projected to reach as high as 85%.
An additional benefit, is that the process for extracting H2 from petroleum hydrocarbons is relatively efficient (compared to current gas refining methods--and I think that was a projected efficiency, since the infrastructure for high quantity H2 production is not in place--I can't find a good source right now, and I got to go...)