It's widely believed that CO2 is the main greenhouse gas contributing to climate change.
Fuel combustion in the engine of your car emits this gas. And the amount of gas emitted is directly related to the amount of fuel used. So the more fuel-efficient your car is, the less CO2 it will produce.
A car's CO2 ranking is obtained from its fuel economy ranking. This ranking is then converted to take into account the type of fuel that the car uses, e.g. petrol (including the octane rating), diesel, LPG etc.
The CO2 ratings are allocated using a similar approach to that used by ADR (Australian Design Rules) for their greenhouse rating.
The more stars a car has for CO2 emissions, the more environmentally friendly it is.
Generally a car that runs on petrol will have the same star rating for CO2 emissions as it does for fuel economy.
Vehicle information specifies the type of fuel a car uses but it doesn't include the octane rating, e.g. 91, 95. Using the average octane rating of fuel sold in New Zealand in 2005, a car that runs on petrol is converted using these equations:
Diesel is converted too - L/100km x 26.050 = CO2 grams/km
L/100 km: Litres per hundred kilometres travelled
CO2 grams/km: Grams of carbon dioxide emitted per kilometre travelled
On vehicle detail pages on this site, you can see the grams/km value below the ratings and what this means in terms of annual emissions of CO2 from the car.
The annual figure is based on a vehicle travelling 14,000 km over a year.