CO2 emissions and g/km meaning

Free car tax and cheap running costs come with low CO2 emissions. Get all the details here, including the meaning of g/km

BuyaCar team
May 18, 2018

Carbon dioxide emissions from your car determine how much tax you pay - it's as simple as that. Switching to a more environmentally friendly car can save you hundreds a year in tax, and in most cases, the same again in fuel.

This is because emissions generally give an indication of how much fuel a car uses and - of course - how much pollution the car is putting into the air. To find out exactly how this affects you, there’s one figure that you need: your car’s emissions in terms of g/km CO2. Here’s how it works


What are CO2 emissions?

When petrol or diesel burns in an engine, carbon dioxide gas - known by its chemical formula CO2 - is produced. The level of emissions indicate how much carbon dioxide is being emitted from the exhaust pipe, and is a good guide to the amount of fuel that a car uses.


What does CO2 g/km mean?

The amount of carbon dioxide in a car’s exhaust gases is calculated using a standard European test and published in every brochure. The results show the average amount of carbon dioxide produced for every kilometre that a car drives.

Carbon dioxide is measured in grams, so the results are written as g/km CO2.

In general, the lower this figure, the less fuel that a vehicle uses: a car with 90g/km CO2, should have good fuel economy. One with 180g/km CO2 will use a lot of fuel.

See the best low emission cars


CO2 emissions in g/km and car tax

At the moment, car tax rates are calculated according to your vehicle’s CO2 emissions. Any car with an official rating of under 99g/km CO2 is exempt from car tax and there are increasing numbers that qualify.

These include cars with efficient petrol and diesel engines, as well as the rising numbers of electric car and hybrid vehicles, which combine an electric motor with a petrol or diesel engine for improved economy.

Cars emitting more than 99g/km CO2 pay tax on a sliding scale from £20 a year to £515.

The rules changed in April 2017. For the first year, you still pay car tax based on your vehicle’s CO2 emissions. After that, all drivers pay a flat rate of car tax under the new car tax rules for 2017. Only vehicles with no CO2 emissions from their exhaust - those powered by electricity or hydrogen - are exempt.

See the best tax exempt cars

CO2 emissions in g/km and company car tax

Choosing a vehicle with low CO2 emissions can save hundreds - or even thousands - of pounds in company car tax. Cars with low g/km CO2 ratings are placed in lower company car tax bands, reducing the level of tax that drivers pay. Click for full details of company car tax rates.

CO2 emissions in g/km and congestion charge

Cars emitting less than 75g/km CO2, which were first registered after January 1, 2011, are curently exempt from the London congestion charge. All electric cars are able to avoid the charge too, as long as the fuel type, listed on their V5C registration document is 'electric'.

You're not able to simply drive into the capital and avoid payment: first you'll need to register your vehicle with Transport for London (TfL), the body that manages the charge. TfL requires a copy of your car's registration document and charges a £10 fee for registration, which needs to be paid each year. You can apply at the TfL Ultra Low Emission Discount site

How to find a car’s g/km CO2 figure

Car brochures - online and paper versions - carry full details of the CO2 emissions of each car. If you’re looking on BuyaCar, you can see the emissions figure for any vehicle by saving it, viewing it in your account and clicking ‘Technical spec’.

A car’s CO2 figure can be affected if you fit larger wheels, or choose a version with four-wheel drive or an automatic gearbox, for example. In some cases, you might find that the difference pushes your car into a different tax bracket.

Is petrol or diesel better for g/km CO2?

Diesel cars are more efficient than petrol versions, as they are able to get more energy out of each litre of fuel. This means that they generally have lower CO2 emission levels, making them cheaper to tax. As they use less fuel, they are often cheaper to run as well, although they can be more expensive to buy in the first place: knowing whether to choose petrol or diesel isn’t always straightforward.

For more than a decade, diesel cars have benefitted from lower car tax rates, thanks to their efficiency. This has helped to boost their popularity, but has also increased the level of other dangerous pollutants in the atmosphere, which are mainly produced by diesel engines.


Are car CO2 emissions dangerous?

CO2 is not a direct risk to human health in the same way as other exhaust gases including nitrogen oxides and tiny pieces of soot called particulates. But it is a greenhouse gas and its contribution to global warming is the reason why governments across the world are forcing car manufacturers to lower CO2 emissions.

Reducing emissions from British vehicles is a major part of the government's plan to cut greenhouse gases in an effort to ensure that global warming does not cause temperature rises of more than 2C higher than pre-industral levels. 

Cheating the emissions test to lower g/km CO2

The recent VW emissions scandal centred on the amount of harmful nitrogen oxides (NOx) produced by diesel-powered cars. But it also highlighted serious issues with the European tests that are used to calculate CO2 emissions, along with emissions of other pollutants and each car’s official fuel economy figure.

The laboratory test (above) was designed decades ago for cars that didn’t have the power or equipment of modern models. The test also contains leeway for manufacturers to legally optimise their cars for the test, including charging up the car's battery to 100% and inflating tyres to high pressures.

It means that emissions and fuel consumption in the real world can be considerably higher than in the laboratory test.

New CO2 emissions test

Since September 2017, a new test used to calculate a car’s emissions has been put in place. This includes a real world section that is carried out on public roads, which measures other harmful emissions, reducing the possibility of being able to cheat the test or take advantage of loopholes. 

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