1 gallon is about 3.8 liters.
Gas is in part cheaper because the US produces a lot itself and only depends to about 40% on foreign oil, where in most countries in europe 99% comes from the outside.
Also, about 5 years ago a gallon of unleaded cost me 88 cents in Texas. In light of this, we doubled and at times more than doubled the cost without increasing taxes on it.
Most of the European cost of fuel is not coming from the manufacturer (although they oversell nicely) but you also have about a 50% taxation on the gas.
see the top 100 gas prices here:
http://www.nationmaster.com/graph-T/ene_gas_pri
and another article that nicely answers your questions:
http://www.caa.ca/e/news-issues/btw/...01-05-10.shtml

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