It seems impossible that a gallon of gasoline, which weighs about 6.3 pounds, could produce 20 pounds of carbon dioxide (CO2) when burned. However, most of the weight of the CO2 doesn't come from the gasoline itself, but the oxygen in the air.

The markup at the pump is about 15 to 20 cts/gal. From that the retailer need to deduct its costs. his net margin is about 3 to 5 cts per gallon sold. an average gas station sell about 4000 gallons per day i.e. an average of 200$ net per day for gasoline retail.