Well, it depends on several factors singularly weight, speed, gas properties and overall optimization in engineering terms. Other ones are not always present although could be equally important.
However it's possible to talk about the normal combustion vehicle like the ones in the image. Thus, in Europe, some years ago, it was calculated to be almost two hundred grams of pollution per kilometer. Actually, some manufacturers are trying to reduce it, but the average is still at 150 grams per kilometer. There are special cases like Lamborghini Aventador LP700-4 reaching almost 400 grams per kilometer like other models from Bentley or Aston Martin. Is this the price of luxury?
How many cars are in this moment traveling kilometers? If you like to calculate the total CO2 emissions right now in the world, try to be relaxed and don't panic!
In the average city there is an estimated number of vehicles that is about 50% of population. Some time ago surpassed a billion globally. Not really rational, but anyway it happens. There's even a star placing much more energy than the one that we could need so the fact is much, much worse.
Several countries are an example of how to live without contaminating, using renewable energy and transportation methods that don't cause atmospheric contamination. Everyone should decide not to cooperate with this facts.