Unfortunately, you, like most people, are not thinking about the total impact of a product, in this case, a car.
To produce a car requires the raw materials to be extracted or produced: rubber, steel, plastic, material for the seats, etc. This is energy intensive.
Then it is put together, again, requiring machines and tools and more energy. Then it is driven on roads, again requiring energy to build and maintain.
Then, there is the petrol, the production of the petrol, from extraction, refinement, shipment, distribution.
Then there is the final disposal of the car.
These are externalities. People aren't used to looking at a product with this holistic viewpoint, considering it's impact from cradle to cradle. A car that produced no CO2 while driving would still have far more environmental impact than a human walking to work. Electric cars, hybrids, these are all environmental dead ends, that assuage people's deep guilt, but still destroy the planet.
I've not even mentioned the social destruction of the automobile, because I've just had my dinner and it would make me feel sick. Cars are inherently un-ecological, and fudging about their CO2 emissions is just pathetic.