Phil is correct but their margins are usually between 5-8%. Which is well in the range is what most companies have as a profit margin. Both Walmart and Exxon have a gross margin of about 25%. But net for Walmart is 4% while it's 7-8% for Exxon.. So their gross profit margin is the same but Exxon is a little better at holding on to it than Walmart.
And Phil is also right about the governent. They make 4x what Exxon makes on a gallon of gas. By the time fees, permits, taxes, registrations, etc etc are all said and done a government body in some form makes up between 40-50% of the price of a gallon of gas.