What you'll see
- CO2 emissions (g/km)
- Fuel type: petrol, diesel, electric, hybrid
- Engine size (cc)
- Year of manufacture
- Euro emission standard indicator
- VED (road tax) band based on emissions
Why do car emissions matter?
Car emissions affect three things that directly hit your wallet: road tax, Clean Air Zone charges, and fuel costs. CO2 emissions determine your VED (road tax) band for cars registered before April 2017, with annual costs ranging from £0 to over £600 depending on the band. For cars registered later, the first-year rate is still emissions-based and can exceed £2,600 for the highest polluters.
Emissions also determine whether your car is compliant with ULEZ and the growing number of Clean Air Zones across UK cities. A diesel car that falls just short of Euro 6 will cost you £12.50 every day you drive it in London, or £8 per day in Birmingham. Over a year of commuting, that is thousands of pounds in charges that a compliant car would avoid entirely.
From an environmental perspective, CO2 emissions indicate how much greenhouse gas a car produces per kilometre driven. Lower-emission cars also tend to be more fuel-efficient, which means lower running costs. A car producing 100 g/km of CO2 will use roughly half the fuel of one producing 200 g/km over the same distance, saving you hundreds of pounds per year at the pump.
If you are comparing similar cars, emissions data helps you make a direct comparison of running costs. Two versions of the same model can have very different CO2 figures depending on engine size, fuel type and whether the car has stop-start technology or a mild hybrid system. Our free check shows you the exact figures for the specific car you are looking at, not just the model average.
Understanding your car's emissions data
The CO2 figure on a car's record determines its tax band, ULEZ compliance and environmental impact. But not all emissions figures are measured the same way.
WLTP vs NEDC: why the numbers changed
Before September 2018, cars were tested under the NEDC cycle, a lab-based test that consistently understated real-world emissions and fuel consumption. From September 2018, all new cars are tested under WLTP, a more realistic cycle. WLTP figures are typically 15-25% higher than equivalent NEDC figures for the same car. This is not because cars got dirtier, but because the test got more honest.
When comparing two used cars, make sure both use the same test standard. A 2017 car's NEDC figure of 120 g/km and a 2019 car's WLTP figure of 140 g/km may represent similar real-world emissions.
The real-world gap
Even WLTP figures overstate fuel economy. Real-world fuel consumption is typically 18-24% worse than the official WLTP figure. Petrol cars overestimate by around 24%; diesel by around 18%. Plug-in hybrids are the worst offenders: their real-world CO2 can be 3.5 times higher than the WLTP figure, because the test assumes a full battery at the start. If you never plug in (as many owners do not), you get worse economy than a conventional petrol car plus the extra weight of the battery pack.
DPF: the diesel trap
Diesel particulate filters (DPFs) became mandatory with Euro 5 (2011 onwards). They trap soot and need regular motorway driving to "regenerate" (burn off the trapped particles at high temperature). Short urban journeys clog DPFs, leading to dashboard warnings and expensive replacements (£1,000-£3,000). Removing a DPF is illegal under the Road Vehicles (Construction and Use) Regulations and will result in an MOT failure.
Frequently asked questions
What are good CO2 emissions for a car?
For a petrol car, under 120 g/km is considered low. Between 120 and 170 g/km is average. Above 170 g/km is high. Diesel cars tend to produce less CO2 than equivalent petrol models but emit more nitrogen oxide (NOx) and particulates. Electric cars produce zero tailpipe CO2. Hybrid cars typically fall between 40 and 100 g/km depending on the type of hybrid system and engine.
How do emissions affect road tax?
For cars registered before 1 April 2017, road tax is calculated on a sliding scale based on CO2 emissions. The lowest emitting cars (under 100 g/km) pay £0 per year, while cars over 255 g/km pay over £600. For cars registered from April 2017, the standard annual rate is a flat £190, but the first-year rate is still emissions-based and can be very high for polluting vehicles. Cars costing over £40,000 new pay an additional £410 per year supplement.
Where does emissions data come from?
CO2 emissions figures come from the manufacturer's official type approval testing, conducted under standardised conditions (NEDC or WLTP test cycles). These figures are recorded with DVLA when the car is first registered. Real-world emissions are typically 15-30% higher than the official figures due to differences between laboratory and on-road driving conditions.
What is the difference between NEDC and WLTP emissions?
NEDC (New European Driving Cycle) was the old emissions test, used until September 2018. WLTP (Worldwide Harmonised Light Vehicle Test Procedure) replaced it and produces more realistic figures because the test cycle better represents real driving. WLTP figures are typically 10-25% higher than NEDC for the same car. Cars registered before September 2018 show NEDC figures, while newer cars show WLTP.
Do emissions affect insurance?
Not directly. Insurance premiums are based on the insurance group rating, which considers factors like value, performance, repair costs and security. However, engine size and power (which correlate with emissions) do affect the insurance group. In general, lower-emission cars tend to have smaller engines and less power, which places them in lower insurance groups. But the relationship is indirect.
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Learn more
Emissions data from DVLA vehicle records and manufacturer type approval testing (NEDC/WLTP). VED rates from HM Revenue & Customs. Last updated September 2026.