trueslayer:
Speaking as one who works in pyrotechnics for a living:
don't be putting hydrogen gas under the hood with HV ignition systems. Hydrogren is very light, and when it gets loose, tends to collect in the ridges of the underside of the hood. It requires exceedingly little energy to ignite. A hydrogen fire of this size takes less than a second, which may be enough to open the hood, and the flash heat can easily ignite the fuel inside the fuel line that will be in close proximity. The car would then become a most unpleasant place to be....
On the practical side, the amount of electrical current required to generate significant hydrogen will very likely drag down the electrical system in the average Ford. When this happens, the alternator goes into full-output mode, which will then require about 8 extra horsepower from the engine to turn. The result, viewed as a system, will be lowered overall gas mileage, due to electrical drag on the underhood system. This effect is frequently demonstrated by those who add 250 watt stereos to their cars, only to then need a larger alternator and battery, followed by more trips to the expensive gas pump.
If, however, all you wish to do is raise your gas mileage, take a look at some of the posts I (and others) have added to this forum on the topic. For instance, my changes have so far added 5 MPG to my wagon's mileage without changing my driving style, and I still have several more tricks up my sleeve (which I'll be posting, as soon as my testing is complete).
One other comment: one of the nation's (U.S.) leading hydrogen-fuel experts is a friend of mine here in Denver. He has been "pushing" hydrogen fueled cars for almost 15 years. His primary comments: hydrogen is dangerous to transport, difficult to start (burn) inside high-compression cylinders and very dangerous when floating around in high-temperature, high-voltage environments (like under hoods). Be careful.