The most serious car makers have different approach to the reduction of toxic emissions:
Improve engine efficiency and fuel combustion with better manufacturing quality (eg. Mazda Skyactive-G engine).
Assist a petrol or diesel engine with a self-recharging battery. These are hybrid cars. (eg. Toyota Auris)
Alternate from petrol to electricity using a battery to charge plug-in from the grid. These are plug-in hybrid cars (eg. Toyota Prius or Volvo V60).
Full electric charge from the grid (eg. Nissan Leaf).
Nox is the exhaust gaz responsible for toxic fog in big cities. It is mainly due to incomplete fuel combustion. Watch this TED-Ed explanation The smog phenomena: https://youtu.be/CdbBwIgq4rs
CO2 emissions are not only to be measured from the tailpipe but also from the power plant if the car uses plug-in electricity. There are major differences per countries depending of the source of energy. Countries like France rely on nuclear power that doesn’t emit C02 but it transfers the risk to an industrial catastrophe. You can read here the electricity emissions per country: http://shrinkthatfootprint.com/electric-car-emissions
Some car makers may use aluminium to build the car body. It makes the car lighter so it consumes less fuel on the road. But producing aluminium requires 20 times more oil than the production of steel. Making an aluminium car is worth 2or 3 years of fuel consumption on the road. So when comparing cars the figures need to be reassessed.
A choice has been made making this table which is to use figures of real world driving made by independent groups of scientist. Please consult section IV if you want read them first hand.
II) Open science meets car industry
Don’t forget that the Volkswagen scandal has a been the result of Open Science which means university researchers have measured and shared their work. It was previously made by car makers only. Open scientists share their techniques and findings online. This new phenomenon has surprised Volkswagen but also others like Fiat or Renault. They cannot alter the figures as much as they wish anymore.
If you want to know more, please consult these articles:
III) Don’t let governments shame you into thinking individuals are the only ones to blame.
Although looking for the cleanest car is an important goal, transportation only accounts for 14% percent of the emissions of toxic gazes. To put this data into perspective , compare this figure to agriculture industry and house heating systems which represent 49% of the emissions altogether. The datas are available on this document: