Why not just convert your existing car?
What doesn't make sense to me, is that a certain famous Daniel Dingle in the Philippines (see his story on Google) has been running his modern standard production cars on hydrogen for more than 30 years. Several Philippine President's even rode in his vehicles.
He was generating the hydrogen on-demand using simple electrolysis, extracting the gas from natural water. The cars engine was almost un-modified.
A certain Steven Meyers in the US (find his story on Google too), also perfected this but was killed while trying to turn it into a commercial interest.
The yahoo group "Watercar" has many members, all people busy developing and using the technology themselves, most though are careful not to say, when they have it working, for fear of a similar end to Mr Meyers.
So why would we pay for Hydrogen? The modifications to the engine are so minor that almost all who read this will be able to do it themselves with items available at their local hardware store.
Some will argue that the amount of electricity necessary to extract the hydrogen is prohibitive, but using a square wave generator instead of an unmodified signal, the amount of current required is significantly reduced to within the scope of a standard car battery and unmodified alternator. The fuel is more powerful and efficient than Petrol or Diesel, causing the engine to run cooler and increasing it's responsiveness.
The exhaust contain zero harmful emissions and is mostly oxygen.
I have the complete plans right here and it really is simple. They can be found online too in varying amounts of completeness.