To add to what GAVol said:
Tighter supply with increased demand + uncertainty in oil producing nations (Nigeria, Venezueal, Middle East) has driven the price of oil up.
Further, there has been a steady increase of regulatory policies including the phasing out of MTBE? which has increased the refining costs.
Finally, there has been a refinery shortfall for years but Katrina and other hurricanes has put an even tighter constraint on refining capacity.
In short, both oil prices and the ability to turn oil into gasoline have come together to drive up prices.
If you want a very thorough explanation read the link below.
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