Because we are first looking at what effects the environment as a whole; global warming, we are only initially interested in which fuel produces the most CO2. The other pollutants only have a very minor (if any) effect on the wider environment, and are more important to the air that we breath. This makes the first part of our investigation nice and simple.
With Diesel and Petrol cars, CO2 levels are published everywhere, and are the main factor in the taxation to the owner and driver of a given car. Ten years ago, it was widely published that Diesel cars produce far lower CO2 levels than Petrol cars, thanks to their greater fuel efficiency. Therefore they became increasingly popular, and have only recently been demonised by Politicians and the media. This is actually largely unfair, because the benefits to the environment are completely genuine. It is regrettable that some vehicle manufacturers felt the need to cheat on emissions tests, which has resulted in some of the recent bad publicity.
So because they are easy to compare, lets first compare Petrol, and Diesel vehicles. We'll pick a small family car, a medium family car, a large family car, and an executive car. This will give us a cross section. I have picked cars made around 2008, so we have a picture from 10 years ago.