generating the hydrogen in the car and then burning it is nothing but a perpetual-motion scam, as nigel already stated.
now, as for carrying along pre-generated hydrogen, it would cost more to buy the hydrogen than the amount of gasoline you would save. If you maybe generated the hydrogen slowly and for free by some renewable power (solar, wind, etc) via electrolysis then it might be somewhat more reasonable, however carrying around hydrogen in your car is dangerous (think hindenburg), and you would need to compress it a lot to carry any significant quantity (think several thousand PSI), thus raising your costs because compressors, storage tanks, solar panels, and all that are NOT going to be cheap, plus carring around a huge, heavy, fiber-wrapped hydrogen tank in your car is probably not going to help your mileage... (and probably be against all sorts of laws and safety regulations...) it's tough to say if you even COULD break even under any circumstances, but I think it's safe to say that you probably won't, at least not in a lifetime.