"A car that burns only half a liter of diesel for a 30 mile commute to work & back would only need the equivalent of about two liters of liquid hydrogen to do the same thing. The published density of liquid hydrogen is about 0.07 grams/milliliter, which is 35 moles per liter. So a 2-liter LH2 tank would contain 70 moles of H2 and at standard temperature & pressure this would occupy a volume of 1712 liters, or 1.712 cubic meters. Not a heck of a lot. That is why I am suggesting that people could manufacture their own fuel for a hydrogen-burning version of the Volkswagon One Liter car. The hydrogen would not even have to be liquefied. A low maintenance pressurized H2 tank should do the trick!"

For a start, a standard modern car has twice that fuel consumption. Second, a range of 30 miles is unacceptable. Even if you don't use it all the time, a range of 100 miles is an absolute minimum for a usable vehicle, and preferebly a multiple of that.

So your fuel tank needs to CONTAIN about 5 times the volume you have chosen - i.e. 8.5 cubic metres (300 cubic feet). (a sphere 6ft 9" across) A pressurised tank needs to be physically strong and thick-walled. One this size would weigh about a ton.

(BTW, your own "not a heck of a lot" tank would be 4 feet in diameter and ~ 1/2 ton in weight)

So you just lost all your load space, and have a 2-seater vehicle that weighs 2 tons, has the performance of a slug, and wastes 50% of its fuel carrying that damn pressure vessel around.

nett fuel economy is NEGATIVE.

All this, to replace a working technology where 40-100 litre gasoline tanks can carry a 4-person vehicle for up to 500 miles with ~3% weight penalty.

You CANNOT replace hydrocarbon fuel in vehicles with hydrogen without major compromises in performance and efficiency.

As with your hydrogen plane. It won't fly.

Your persistence with this notion demonstrates the calibre of your thought processes and ability to learn.