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Cars & travel.

Updated: 20 Sep 2021

Car CO2 emissions explained

What you need to know about CO2 plus the big fines manufacturers are facing, how CO2 emissions affect your road tax and our free to use CO2 checker.
Adrian Porter
Car emissions from petrol cars

We don’t rely on official car CO2 figures. We measure CO2 ourselves in our own tests, that go further than the official ones, to produce more realistic figures.

Our cars CO2 emission checker below shows two different measurements of CO2:

  •  The first is the amount of CO2 that come out of the exhaust pipe. This is also known as tank-to-wheel (TTW) CO2, and is the simplest way to measure CO2. Electric cars and hydrogen fuel cell cars will not have TTW figures, as they do not emit CO2 (electric cars don't even have an exhaust).
  • The second CO2 measure is our well-to-wheel (WTW) CO2 figure. This takes account of the CO2 impact of generating the fuel, whether it be petrol, diesel, electricity or hydrogen, and looks at how efficiently the car uses that fuel.

You can use our WTW figures to directly compare CO2 from all cars, no matter what fuel it uses.

Free car CO2 checker: check by make and model

  • Note: The data contains WTW CO2 figures for all cars we’ve tested since 2012, and TTW CO2 figures from cars tested since 2017. We will be adding TTW for all cars at a later point.
  • Our test programme was updated in 2017, and figures are not comparable to cars tested on the 2012-2016 programme.

Our unique lab and road tests reveal the very best electric cars for 2021.

CO2 and car tax

For cars first registered after April 2017, the amount of CO2 your car emits defines how much car tax, also known as VED or vehicle excise duty, you pay in the first year of ownership.

You’re actually unlikely to notice this, as the cost is wrapped into the on the road price. Then, from the second year of ownership onwards, you pay a standard rate regardless of CO2 output. The standard rates are currently: 

  • £155 for regular petrol/diesel cars 
  • £145 for alternative fuel cars like hybrid cars
  • Zero emission cars like electric cars and hydrogen fuel cell cars are still exempt. 

However, if you paid more than £40,000 for your car, you can expect to pay an additional £335 on top of the standard rate, for five years.

This £40,000 rule (also referred to as the 'expensive car supplement') originally applied to zero emission cars, but no longer does as of April 2020.

If your car was first registered before April 2017, the amount of car tax you pay every year will be defined by its official CO2, and the yearly rates are also linked to inflation.

Any car that produced less than 100g/km of CO2  that was first registered before April 2017 is tax free, and under current rules will remain tax free for its entire life.

That means an older, less efficient hybrid car will be exempt from car tax, but a newer, more efficient hybrid won’t be.

Confused about car tax? Head to our car tax guide to find out more.

CO2 rules and big fines

CO2 is not part of the same emission standards as NOx, PM and CO (ie Euro 6d). You can read more about emission standards in our guide to low emission cars.

Instead, cars are technically allowed to produce as much CO2 as they want, with one important caveat: across all the cars that a single manufacturer sells in a year, the average CO2 figure from official tests, from all the cars sold, must equal or be less than a limit set by the EU.

In 2019, that limit was 130g/km. So across all cars sold across the EU by a single manufacturer, the average CO2 from cars that manufacturer sold had to equal 130g/km or less.

But as of 2021, that limit dropped to 95g/km. That’s also going to be extremely challenging for carmakers. As the UK left the EU at the end of 2020, it actually has its own 95g/km limit for car makers.

That means all cars sold in the UK, per manufacturer, will have to average 95g/km, and all cars sold in the rest of the EU will have to, separately, also average 95g/km or less.

2020 was both the transition period for leaving the EU and, separately, being treated as a transition period from 2019’s 130g/km limit to 2021’s 95g/km limit.

If the cars sold by a manufacturer exceed the limit, it will be charged €95 for every single gram per km higher than the average they are, multiplied by every car the manufacturer sold that year.

Cars with high emissions in our tough tests, which are more stringent than official tests, can't be Best Buys. Only cars that are economical and reliable make the grade - see our pick of the best cars.

CO2 will change the EU/UK car industry

Average CO2 values from car emissions in the UK have actually risen for the third year running. According to figures from the SMMT (Society for Motoring Manufacturers and Traders), the average 2019 figure was 127.9g/km across all cars sold in the UK - this is a rise of 2.7% compared to 2018.

So in order to meet the 95g/km, expect some massive changes to occur across the industry, including the removal from sale of some higher CO2 cars in the next couple of years.

You’ll also see a lot more electric cars going on sale and, despite the upcoming ban on petrol, diesel and hybrid cars, a lot more plug-in hybrid cars.

There's a special reason for this. Not only do electric cars have zero tailpipe emissions (and are referred to as zero emission vehicles, along with hydrogen fuel cell cars), but any car that produces less than 50g/km of CO2 in official tests, during 2020, is actually counted as two cars when it comes to calculating the CO2 average. These car sales are referred to as ‘super credits’.

Under current plans, each sub-50g/km car will count as 1.67 cars in 2021, and 1.33 cars in 2022. This is before super credits are eliminated from 2023 onwards.

Manufacturers will also be allowed to form ‘pools’ with each other and create a combined CO2 average between them, placing high values on Tesla and manufacturers with a larger number of electric or low emission cars.

When UK-only limits take effect in 2021, super credits and manufacturer pools will be allowed. UK specific pools can be set up by manufacturers.

If an electric car isn't right for you, we'll help you choose your next model - see our expert advice on petrol vs diesel vs hybrid.

Rise of the mild hybrid

In the pursuit of lower CO2 figures, we’re currently seeing a wide roll out of mild-hybrids, such as the mild hybrid version of Volvo’s XC90 SUV (pictured above).

Mild hybrids have a small battery-powered electric motor in addition to the main combustion engine. But unlike regular hybrids, the electric motor in a mild-hybrid does not power the wheels directly.

Instead, the electric motor assists the combustion engine, either adding a boost for extra power when needed or taking some of the strain on the engine for better fuel efficiency.

Mild-hybrids are essentially combustion cars with a little bit of help. But you may want to skip this phase and head straight for a hybrid or plug-in hybrid instead.

To find out which hybrids and plug-in hybrids we rate, based on our tough lab and road tests, go to the best hybrid cars.