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Metal is permeable to air in tires. Alloy wheels loose air through the metal (supposedly, though I've never actually observed the phenomenon myself—car parked for 4 years still has some air in tires). One of the big obstacles of hydrogen as a fuel is having to store it in pressurized containers to get range out of it, unless that guy who claims to have come up with a way to store it in metal (aluminum foam?) is legit.
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