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CO2 emissions from cars? A beginner’s guide

Since the invention of the internal combustion engine in the mid/late 1800s, road vehicles have become vital to most economies around the world. People and goods can be transported at will to virtually any location on the planet (within reason).

However, carbon dioxide (CO2) – a by-product of these engines – is one of the main contributors to climate change. In turn, the transportation industry (including trucks, lorries, buses, vans, cars, motorcycles, etc.) contributes significantly to the problem, although it’s far from the only one.

This article is a beginner’s guide explaining what CO2 emissions from cars are, how they’re measured, and the UK rules and regulations surrounding the issue.

What is CO2 and what’s the big deal?

Despite its current bad reputation, carbon dioxide itself is vital to all life on Earth. It’s a gas found in our atmosphere compromised of one part carbon (“C”) and two parts oxygen (“O2”). Plants "breathe" it in, and thus the carbon – an essential component of a living organism – enters the food chain.

NASA estimates that before the Industrial Revolution began, the concentration of CO2 in the atmosphere was about 280 ppm (parts per million). By the year 2000, we’d hit 370 ppm, and we’re now well over 400. These numbers are rising at an alarming rate and are indisputably linked to human activity-based emissions.

The problem, then, isn’t carbon dioxide itself – it’s the massive excess of it we’ve produced. Check out this research by Hannah Ritchie and Max Roser for a somewhat accepted view of which industries contribute the most greenhouse gases to the atmosphere. Road transport is estimated to give out about 12% (60% of which is passenger vehicles like cars).

What is the greenhouse effect?

Again, despite the terrible connotations associated with “greenhouse gases”, the greenhouse effect is vital for our survival on planet Earth. Without our atmosphere (the “greenhouse”), the sun’s powerful solar energy would strip all oxygen away. The heat absorbed from the sun’s rays would also completely evaporate at night, freezing us. In short, without the greenhouse effect, we’d be dead.

CO2 makes up about two-thirds of our greenhouse gas emissions, the other leading players being methane and nitrous oxide. As we release more of these than the planet’s natural rhythm enjoys, they gather in the upper atmosphere and absorb more of the sun’s heat. As a result, Earth warms up, leading to floods, rising sea levels, unpredictable freak weather and less reliable food production.

How are CO2 emissions from cars produced?

Passenger vehicles are often vilified as one of the leading causes of global warming – and it’s true, to some extent. CO2 emissions from cars represent approximately 7.1% of all worldwide annual greenhouse gases. If most cars on the road were electric (and we produced the electricity from a clean source), this would remove a significant chunk of our pollution output.

In an internal combustion engine, fuel (petrol or diesel – a product of crude oil) mixes with the air. This mixture is then ignited by a spark (or compression in a diesel engine). Within the combustion chamber, several reactions occur, but the one we’re most interested in, from a carbon dioxide point of view, is:

CXHY + O2 → CO2 + H2O

Fuel + Oxygen → Carbon Dioxide + Water

The fuel is made up of hydrocarbons, CXHY. These molecular chains mix with the oxygen and nitrogen in the air before igniting. Once the air/fuel mixture has ignited and driven the engine round, we have no use for the resultant chemicals, so they get pushed out the exhaust pipe and into the air.

If you would like to wrok out the emissions from your vehicle, you can try out our CO2 emission checker.

How are a vehicle’s carbon dioxide output levels measured?

Every new car on sale in the UK is required, by law, to display its fuel efficiency and CO2 emissions – both of which are very important from an environmental point of view.

CO2 emissions are most often measured in grams per kilometre (g/km). This reading indicates how much carbon dioxide the car’s exhaust puts out compared to how far you drive. You’ll find your vehicle’s average carbon dioxide emissions rating in the V5C log book.

The most recent vehicle tax updates, from April 2017, encourage all drivers to move away from petrol and diesel, turning instead to electrical vehicles (EVs). The UK government will undoubtedly continue to push the target lower and lower.

During an emissions test or an MOT, the technician places a specialist probe into your car’s tailpipe. This device measures the pollution contents of the exhaust gases, including carbon dioxide. Depending on your vehicle’s age, the system will either accept (pass) or reject (fail) these levels.

Electric vehicles and carbon dioxide

Electric vehicles still technically lead to carbon dioxide emissions, although not in the same way. While they come with many benefits, they aren’t yet a permanent solution.

For example, we get the precious metal needed for batteries from massive mines before they’re shipped across the world. An old battery is also virtually useless. And on top of all that, we must assume the grid generates the electrical power to charge the EV using a clean, sustainable method. Otherwise, we’re simply changing the CO2 output from the exhaust to the power station.

Instead, some studies measure EV carbon dioxide emissions from production through to the end of life. These levels are notably less than a standard petrol or diesel car, but unfortunately, they’re still quite high. As a result, the most important benefit of electric vehicles is reducing tailpipe pollution, especially in busy cities.

While they aren’t yet the solution to climate change, they present a path with a good chance of success.

Recent and upcoming UK government legislation regarding carbon dioxide from cars

In the UK, we pay road tax based on the CO2 emissions from the car’s exhaust. It’s obviously designed to encourage us to move away from heavy fossil fuel-burning vehicles and swap to cleaner alternatives.

Road tax was once indicated by the tax disc displayed on the windscreen. These days, it’s all electronic – you can remove that old piece of circular paper and throw it away if you haven’t already!

It’s also on a per owner basis. The road tax doesn’t carry over from the previous owner, so as soon as you buy a new car, you’ll need to tax it. You can also get a tax refund for your old car by telling the DVLA you no longer own it. They’ll refund you the appropriate amount.

In the lowest tax band, A, you don’t pay any road tax at all – although you still need to go to the DVLA’s website to “purchase” it. These are the cleanest vehicles on our roads.

If you drive a car that puts out significant pollution levels, you could be paying many hundreds, if not thousands, per year in road tax.

New road tax laws from 2017

The rules changed for new cars registered from 1 April 2017, and the tax brackets shifted. Now, you’ll pay a one-off charge when first buying and registering a new vehicle. This is followed by a yearly, bi-annual, or monthly (depending how you pay) payment. The initial upfront charge can be as high as £2,365 if your vehicle emits over 255 g/km of carbon dioxide.

You don’t have to pay any road tax on all electric cars since they have no direct carbon dioxide emissions. Check out the complete list of tax bands here.

From 2021, the UK government imposed a maximum carbon dioxide emission level of 95 g/km per new car, encouraging manufacturers to reduce their CO2 outputs across their ranges.

Overall, it’s clear that we’re being encouraged to reduce our polluting methods of transport as much as we possibly can.

Can I reduce my car’s carbon dioxide emissions?

It’s possible, but very unusual, to change your vehicle’s tax classification. Car drivers can do so if they swap the engine out for a different one or convert it to an alternative fuel type, including electric. Learn more on the UK government’s website.

As such, even though you can take several steps to lower your car’s CO2 emissions, you’ll probably still be paying the same rate to the government each year. The following tips help reduce your vehicle’s carbon dioxide emissions, increasing fuel efficiency simultaneously.

  • Drive smoothly.
  • Accelerate and brake gently.
  • Drive slower.
  • Use high-octane fuel.
  • Use minimal electrical appliances.
  • Keep windows shut and remove roof racks and any unnecessary weight.
  • Ensure tyres are properly inflated – not too high, not too low.

You can learn more about how to improve your fuel efficiency in our previous guide.


The need for transportation is virtually inescapable in our modern world. We need to cross significant distances to go to work, meet friends and family, collect goods, or go on holiday.

At the time of writing, we see fuel prices reaching record highs. EVs are beginning to gain momentum in the market; a potentially promising sign.

Climate change is perhaps more real than many people realise or admit. The most impactful change we can make isn’t just to switch to cleaner modes of transportation but to make a lifestyle change. Travelling in groups, walking or using bicycles, using fuel-efficient vehicles and driving more carefully will all help reduce CO2 emissions from cars and other forms of transportation.

Despite well over a century of relatively reliable service, the internal combustion engine is on the way out. In the future, as our roads become cleaner, expect to see the focus shift dramatically to more and more sustainable energy sources, such as tidal, wind and solar.