Which? warns of rising CO2 emissions from latest cars

Which? test data reveals that the latest cars on the road in the UK produce more carbon dioxide (CO2) than older vehicles, despite the country’s 2050 target for net-zero carbon emissions.


Following an analysis of its car testing results, Which? has found that the most recent petrol, diesel and hybrid cars are producing seven per cent more CO2, on average, than older vehicles.


The consumer group looked at every car it has tested since the start of 2017 (292 models) finding an average increase in CO2 of 10.5g/km – a seven per cent increase – from 151.6g/km to 162.1g/km. The rise in CO2 emissions was seen across almost all car classes and fuel types.


The results revealed cars that have been certified under official tests and meet the latest emission regulations, Euro 6d-temp and Euro 6d respectively, produce more CO2 in Which? tests. This is compared with cars that were certified under the previous emission regulations, Euro 6b and Euro 6c, which lasted until 2017 and 2019 respectively.


Small petrol cars (Ford Fiesta / Volkswagen Polo size*) saw an average increase in CO2 emissions of 11.2 per cent (from 131g/km to 145.7g/km), while mid-size petrol SUVs (Nissan Qashqai size*) rose by 20.4 per cent (from 157.6g/km to 189.8g/km*). Worse than either of these categories were large petrol-hybrid cars (BMW 3 Series size*), which recorded an average increase of 31.7 per cent (from 89.2g/km to 117.4g/km).


All car models analysed were subjected to exactly the same tests, in lab-controlled conditions. Which? tests are more realistic than the official tests, as cars are run through an additional tough motorway cycle where vehicles are accelerated up to and sustain motorway speeds. Additionally, cars are loaded with 200kg, with the air-con and radio switched on – to make sure it’s set up for everyday use.


One reason that Which? experts cited for the increase in CO2 emissions in the latest models could be the weight of the car. Carmakers have invested in lightweight materials for car construction, but the increasing size of vehicles and additional tech contributes to an overall weight gain. Which? tests have revealed an average increase of 3.4 per cent (67kg) in cars that meet Euro 6d-temp/6d, compared with earlier models that meet Euro 6b/6c.


Which? lab experts added that the technical and software modifications needed to reduce harmful emissions could have led to a rise in fuel consumption and, as a result, higher CO2 emissions.


The consumer champion’s analysis did find emissions linked to harming human health – NOx (oxides of nitrogen) and CO (carbon monoxide) – are now a fraction of what they were, compared to cars made in recent years. In petrol cars, the level of carbon monoxide emissions has dropped by 40.6% per cent, on average, when comparing newer cars tested under Euro 6d-temp or Euro 6d versus older cars tested under Euro 6b or Euro 6c. Across all diesel vehicles, the decrease in NOx was an average of 84 per cent.


From 2021 onwards, the average CO2 from official tests must be 95g/km or less (the limit in 2019 was 130g/km) across all cars that a manufacturer sells in a year, with fines of 95 euros per gram over the limit for every single car sold.


Which? will continue to call out and penalise manufacturers that produce high-polluting cars by denying them the Which? Best Buy status.


Lisa Barber, Editor of Which? Magazine, said:

“It is shocking to see our tests uncover increasing levels of carbon dioxide emissions for the latest cars that are being built and sold to UK consumers.


“Manufacturers must ensure that they are doing everything in their power to create cleaner vehicles that are fitter for our planet and its future.”


Table of car emissions (by vehicle)


Type of vehicle Small cars (petrol) Mid-size SUVs (diesel) Mid-size SUVs (petrol) Large cars (petrol-hybrid)
CO2 rise +11.2% +6.4% +20.4% +31.7%
CO fall -40.6% -72.3% -47.4% -31.1%
NOx fall -43% -85% -26% -61%


 *Cars mentioned for size reference only.  




  • In January 2020, Which? analysed the results of its tests for every car that it has tested since 2017 – a total of 292 models – to uncover the differences in car emissions.
  • Which? now offers more emission information to individual car reviews. It has a new tool on which.co.uk/emit that shows you the levels of vital pollutants from every engine it has measured since 2012. If you use information from this tool, please credit Which? as the source of the data.
  • Please find a video to accompany this story here.

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