Actually, Exxon's take - it's net profit - is more like 8.75 cents on a dollar; or at $4.00 a gallon, more like 35 cents a gallon.
But, your point is still a good one, in as much as the total state and federal tax take on a gallon is, on average, over 35 cents a gallon. It averages 49.5 cents nationally, between a high of 61.3 cents in the West to a low of 38.5 cents in the South.
Download the May 2011 Summary at:
http://www.api.org/statistics/fueltaxes/
Their biggest profit centers are in lubricants; does the 8% number come from a extrapolation of a quarterly report? Chevron was extremely solid for a number of years for profit on gasoline - 2.5% net margin (or 10 cents at 4.00)