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As others have said, hydrogen is not an energy source, but an energy storage medium. Unfortunately, it is about the worst possible storage medium for cars. It might beat giant rubber bands, but not by much.
You start out with something that's inefficient to create. Then add the fact that it's extremely difficult to store. Because the H2 molecule is so small, it seeps through almost anything, often changing the material structure as it goes. (See "hydrogen embrittlement".) To collect a useful quantity, you either have to compress it to high pressures, which takes energy and strong, heavy storage tanks, or you have to liquify it. That again takes energy to cool it to the super-low temperatures needed, and more energy to keep it cold. Then you have to add all this infrastructure to trucks or pipelines...
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