Cars and greenhouse gases

While we're on the subject of my not being too hot at science, here is what, to me, is a conundrum. The world's least green car, the 8.3 litre Dodge Viper, is said to do between 13 and 22 miles per gallon and according to the US Environmental Protection Agency produces 11.4 tons of greenhouse gases when driven over the American average of 15,000 miles in a year.

Now, even at the worse end of the consumption figures, the car will have used 1,154 gallons of petrol which weigh 8,424 lb or 3.76 tons. The law of conservation of mass tells us that if 3.76 tons go in, no more than 3.76 tons can go out.

Aha! you say, but it has mixed with air (nearly 8 tons of it). But air is harmless. This is like saying that if you dilute your wine with the same amount of water you have drunk twice as much alcohol.

So what is going on here? Anyone know?