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New cars emit more climate changing CO2 than old, according to Which? tests

On the plus side, they also emit fewer human-harming pollutants - we may have saved our lungs at the expense of the planet

New cars emit more climate changing CO2 than old, according to Which? tests

Our car emission tests have revealed that the latest cars are producing more CO2 (carbon dioxide) than the older ones they’re replacing – on average, 7% more (10.5g/km). 

We’ve looked at every engine of every car we’ve tested since 2017 (when we last changed our test programme). That’s 292 models. All were subjected to exactly the same test programme in lab-controlled conditions – our tests are more realistic and tougher than the official tests for numerous reasons, including the fact that we include a motorway cycle.

When we compared cars certified under official tests that meet the very latest emission standards (Euro 6d-temp and Euro 6d) with cars certified under the previous regulations (Euro 6b and Euro 6c), we found the newer cars produce more CO2 in our tests.

But it’s not all bad news. Our tests have also discovered that the latest emission standards have done a fantastic job of reducing human-harming air pollution.

Compared to cars that meet slightly older emission standards, we’ve seen an 84% fall in NOx (oxides of nitrogen) and CO (carbon monoxide). NOx, along with particulate matter (PM), is thought to be responsible for tens of thousands of premature deaths here in the UK every year.


Use our free tool to find out how dirty or clean a car really is, plus discover our top five cleanest and dirtiest cars – go to low emission cars.


Video: new cars and CO2 explained

Good for our lungs, bad for the earth

Our results come from analysing every engine of every car we’ve tested since the start of 2017, which is when we updated our test programme.

All 292 models we looked at have been subject to exactly the same tests, in lab-controlled conditions.

What we found is that the fall in NOX and CO pollutants and rise of CO2 is just about universal across car classes and fuel types. We’re seeing CO2 rises from petrol-hybrid hatchbacks, to diesel SUVs to small petrol-powered cars.

Among the petrol-powered cars from the small car class (think Ford Fiesta/Volkswagen Polo sized cars), the average rise from Euro 6b/6c to Euro 6d-temp/6d was 14.7g/km, or 11.2%, while NOx came down by 43% and CO fell by 40.6%.

Here are some fuel types and car class examples:

Why are newer cars producing more CO2?

This rise might be down to an issue related to the very technology that is actually reducing NOx and CO, and is hopefully a temporary one.

Our lab experts say the technical and software modifications needed to reduce emissions, and therefore adhere to the latest emissions standards (we explain the car emissions standards in detail here) have led to an inevitable rise in fuel consumption and CO2.

However, they also say that CO2 levels in our tests could drop again as manufacturers continue to make improvements.

Another possible reason for the increase in CO2 emissions could be the weight of the car, as that will have an impact on CO2 and fuel economy.

Carmakers have invested in lightweight materials for car construction, but increasing the actual size of the car and adding more tech contribute to an overall weight gain.

We know that cars have gained weight as we weigh every one we test. We can report we’ve seen a 3.4% weight increase (67kg) for cars that meet more recent emission standards (Euro 6d-temp and 6d), compared with those meeting older standards (Euro 6b and 6c).

Can I still buy a low emission car?

Yes. In spite of this universal trend, there are still cars we’ve tested that have genuinely low levels of NOx and CO while keeping CO2 emission to a minimum.

We’ve listed the five cleanest and the five dirtiest cars we’ve tested since updating our test programme, and you can use our free tool to scroll through all the cars we’ve tested to see how clean, or not, your car really is – go to low emission cars.

The rise of official CO2

Completely separate to what we have found in our tests, the amount of official CO2 emitted from cars in the UK has also risen.

The Society of Motoring Manufacturers and Traders (SMMT) revealed earlier this year that the average amount of CO2 emitted by cars had risen for the third year in a row – by 2.7% to 127.9g/km.

This will be affected by a change in official test procedure. This has changed from the outdated and unchallenging  NEDC tests, to tougher WLTP tests. So, naturally, official recorded emissions will rise as the newer WLTP tests are more representative.

But this doesn’t explain the difference in our tests, as our test programme hasn’t changed.

Also, as this average is based on official CO2 from cars that have been sold in the UK, it’s going to be impacted by the nation’s swing away from diesel cars following the VW diesel scandal (dieselgate).

This is because diesel cars produce less CO2 than petrol. So if more people switch to petrol cars, then official CO2 will rise.

What’s happening to reduce CO2 from cars?

Tough new CO2 limits mean manufacturers will have to reduce average CO2 to 95g/km, or face incredible fines.

This is an extremely challenging target and one that will inevitably change the car market, from increased deals on electric cars and hybrids, to the possible removal of high CO2-emitting cars.

Find out more about CO2, the fines and the changing face of the UK car market by heading to our guide on CO2 from cars.

Our free CO2 emissions tool

Our car CO2 emissions guide also contains our free-to-use CO2 emissions tool.

It shows CO2 in two ways:

  • The amount we’ve measured from cars in our tests
  • Our well-to-wheel (WTW) calculations. These look at the CO2 impact of generating and delivering the fuel, as well as how efficiently the car uses that fuel.

It means you can use our CO2 tool and WTW values to directly compare cars for all fuel types – from electric cars and hydrogen fuel-cell cars (like the Toyota Mirai), to hybrid, petrol and diesel cars.

We don’t recommend cars that have produced high amounts of toxic emissions in our tests. Skip straight to what we recommend with our best cars of 2020.

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