CO2 is what comes out of your exhaust when fuel burns

When petrol or diesel burns in your engine, it combines with oxygen and produces carbon dioxide (CO2) as a byproduct. Every litre of fuel you burn creates roughly 2.3 kilograms of CO2 that exits through your tailpipe. This is not a pollutant in the way that nitrogen oxides or particulates are—it will not damage your lungs or make your car fail an emissions test. But CO2 is a greenhouse gas, meaning it traps heat in the atmosphere and contributes to climate change.

Your car's CO2 emissions are directly tied to fuel consumption. A car that uses less fuel produces less CO2. This is why fuel economy and CO2 output are essentially the same measurement expressed two different ways: one tells you how far you can drive on a litre, the other tells you how much carbon dioxide you release per kilometre driven.

Key Takeaways

  • CO2 emissions from your car are measured in grams per kilometre (g/km) and depend entirely on how much fuel your engine burns.
  • Your vehicle's official CO2 figure comes from a standardised test, but real-world emissions are usually higher because driving conditions vary.
  • Newer cars emit less CO2 than older ones because engines are more efficient, but the difference between a petrol and diesel car of the same model is small.
  • Reducing your own CO2 output means driving less, maintaining your car properly, and avoiding unnecessary weight and idling.

How CO2 emissions are measured and reported

Manufacturers test new cars in a laboratory under controlled conditions to determine their official CO2 figure. In Europe, this test is called WLTP (Worldwide Harmonised Light Vehicle Test Procedure). The car runs through a set driving pattern at fixed speeds, and the emissions are measured and converted to grams per kilometre. This number appears on the vehicle's window sticker, in the owner's manual, and in registration documents.

The official figure is useful for comparing one car to another, but it does not match what most drivers actually experience. Real-world driving involves acceleration, braking, motorway speeds, and traffic—all of which use more fuel than the test cycle. Studies consistently show that actual CO2 emissions run 20 to 40 percent higher than the official figure, depending on how and where you drive.

Why newer cars produce less CO2 than older ones

Engine technology has improved significantly over the past two decades. Fuel injection is more precise, combustion is more efficient, and engines are lighter and smaller. A 2024 petrol car typically emits 30 to 50 percent less CO2 than a 2004 model of similar size, even though both burn the same type of fuel.

Diesel engines are slightly more efficient than petrol engines—they convert more of the fuel's energy into motion—so a diesel version of the same car model usually produces about 15 to 20 percent less CO2. However, this advantage is smaller than many people assume, and diesel's other emissions (nitrogen oxides and particulates) are why diesel cars now face restrictions in many cities.

The difference between CO2 and other car emissions

CO2 is only one of several things that come out of your exhaust. Nitrogen oxides (NOx), particulate matter, and unburned hydrocarbons are pollutants that damage air quality and human health. These are what emissions tests actually measure and regulate. Your car must meet strict limits on NOx and particulates to pass an MOT or emissions inspection.

CO2 is not regulated by emissions tests because it is not a local air pollutant—it does not make the air around you unhealthy to breathe. Instead, CO2 is regulated through fuel economy standards and vehicle tax. In the UK, vehicle tax rates are linked to CO2 emissions, so cars that emit less CO2 pay lower tax. This creates a financial incentive for manufacturers to build more efficient cars and for buyers to choose them.

What affects your car's real-world CO2 output

Your driving habits have the largest effect on how much CO2 your car actually produces. Aggressive acceleration and hard braking both waste fuel. Motorway driving at high speeds uses more fuel than town driving at the same distance because air resistance increases with speed. Idling—sitting stationary with the engine running—burns fuel and produces CO2 without moving you anywhere.

Vehicle maintenance also matters. Under-inflated tyres increase rolling resistance and force the engine to work harder. A clogged air filter restricts airflow and makes combustion less efficient. Worn spark plugs or a faulty oxygen sensor can increase fuel consumption by 10 to 20 percent. Carrying unnecessary weight—roof racks, tools, or cargo you do not need—forces the engine to burn more fuel to move the same distance.

The type of driving you do affects emissions more than the type of car you own. A driver who accelerates smoothly, maintains steady speeds, and keeps their car well-serviced will produce significantly less CO2 in an older car than a driver who accelerates hard and ignores maintenance in a new one.

CO2 emissions and vehicle tax in the UK

Vehicle tax (road tax) is calculated based on a car's CO2 emissions figure. Cars registered after April 2017 pay a flat rate after the first year, but the first-year rate depends on emissions. A car emitting 100 g/km pays less in year-one tax than one emitting 200 g/km. This system encourages buyers to choose lower-emission vehicles.

Electric vehicles pay no vehicle tax because they produce zero tailpipe emissions. Hybrid vehicles (which combine a petrol engine with an electric motor) produce less CO2 than petrol-only cars because the electric motor handles low-speed driving and the engine shuts off when not needed. However, the CO2 savings depend on how much you actually use the electric mode—a hybrid driven mostly on motorways at high speeds will not save as much fuel as one driven mostly in town.

How to reduce your car's CO2 emissions

The most direct way to reduce emissions is to drive less. Every kilometre you do not drive produces zero CO2. If you can walk, cycle, or use public transport for some journeys, that is the single most effective change you can make.

If you must drive, maintain your car properly: check tyre pressure monthly, replace air filters on schedule, and keep up with servicing. Drive smoothly—accelerate gradually, anticipate stops, and avoid idling. On motorways, stay at 60 to 65 mph rather than 70 mph; fuel consumption rises sharply above 65 mph because of air resistance. In town, plan your route to avoid congestion and unnecessary stops.

When you are ready to replace your car, choose one with lower official CO2 emissions. The difference between a 120 g/km car and a 180 g/km car is substantial over the car's lifetime. If you drive high mileage, a hybrid or electric vehicle will produce significantly less CO2 than a conventional petrol car, even accounting for the electricity used to charge it.

Frequently Asked Questions

Is CO2 the same as the emissions my car fails an MOT for?

No. An MOT emissions test measures nitrogen oxides, particulates, and unburned hydrocarbons—pollutants that damage air quality. CO2 is not tested because it is not a local pollutant. Your car can pass an MOT and still emit high levels of CO2 if it is inefficient.

Does a diesel car really emit less CO2 than a petrol car?

Slightly, yes—usually 15 to 20 percent less for the same model. But diesel engines produce more nitrogen oxides, which is why many cities now restrict them. The CO2 advantage is smaller than most people think, and it only matters if you drive high mileage.

Why is my car's real CO2 output higher than the official figure?

The official figure comes from a controlled laboratory test that does not match real driving. Acceleration, motorway speeds, traffic, and cold starts all use more fuel than the test cycle. Real-world emissions are typically 20 to 40 percent higher depending on how and where you drive.

Can I reduce my car's CO2 emissions by changing how I drive?

Yes. Smooth acceleration, steady speeds, proper tyre pressure, and regular maintenance can reduce fuel consumption by 10 to 25 percent. Avoiding idling and unnecessary weight also helps. These changes lower your emissions and save you money on fuel.

Do electric cars really produce zero emissions?

Electric cars produce zero tailpipe emissions, but the electricity that charges them comes from the grid, which still uses some fossil fuels in most regions. However, even accounting for grid emissions, an electric car produces roughly 50 to 70 percent less CO2 over its lifetime than a petrol car driven the same distance.