What are a car's CO2 emissions?
When petrol or diesel burns in an engine, carbon dioxide gas – also known as CO2 – is the result. The amount of carbon dioxide that is emitted from the exhaust, measured in grams per kilometre (g/km), is a fairly accurate guide to the amount of fuel that your car is using.
How much you pay in road tax (or vehicle excise duty, to use its correct name) depends on the amount of CO2 emitted by your car. In other words, the more carbon polluting your car is, the more you’ll pay in tax.