I tried googling this set of keywords:

cost of electrolyzing water to produce hydrogen equivelent to a gallon of
gasoline 

and got a bit of a surprise,

What with the manufacturing of new equipment and improved efficiency
and lowered costs as well as the end to endlessly paying more 
for foreign oil while sending our dollars overseas...

Of course much is made of the difficukty of transporting the stuff but why 
not produce it locally from available technology?

What's the downside
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