Nature, fossil fuels, and global warming

[Shiva]

In Nature, fossil fuels, and global warming: How many trees should I plant to balance my yearly CO2 output? Cecil says

Consider two of the carbon calculators I found on the Web. Both agree that driving a medium-size car 10,000 miles a year will generate roughly four tons of carbon dioxide annually

Now if gasoline weighs 6 pounds per gallon and the average medium-size car gets 20 miles per gallon that means that in 10,000 miles of travel the car will burn 500 gallons or 3,000 pounds of gasoline.

How in the world does a ton and a half become four tons?

Has Cecil been snookered by tree huggers? Alternately, is my math really that bad?

[bup]

Like I said in another thread, 60% of the weight of exhaust comes from air, not the octane in the gas. When you burn gas, you combine oxygen with carbon.