Understanding CO2 emissions UK — carbon dioxide (CO2) is the primary factor determining your road tax. Here is everything you need to know about vehicle CO2 and how it works in 2026.

What Is CO2 and Why Does It Matter for Road Tax?

CO2 is the greenhouse gas produced when fuel is burned in an engine. The UK government uses CO2 emissions as a proxy for environmental impact, linking it to Vehicle Excise Duty. The more CO2 your vehicle produces, the more you pay in road tax. This creates a financial incentive to choose lower-emission vehicles.

How Is CO2 Measured?

CO2 is measured in grams per kilometre (g/km) through standardised laboratory testing on a rolling road. The vehicle is tested through a specific drive cycle that simulates urban, rural, and motorway driving. The result is a combined figure — not separate urban/motorway figures.

WLTP vs NEDC: Why Your CO2 May Have Changed

Since 2018, all new vehicles are tested under the WLTP (Worldwide Harmonised Light Vehicle Test Procedure), which is more realistic than the previous NEDC test. WLTP figures are typically 15-30% higher than NEDC figures for the same vehicle. This means a car registered from 2018 onwards may have a higher official CO2 figure than pre-2018 equivalents.

Real-World CO2 vs Official Figures

Official WLTP CO2 figures are laboratory results. Real-world driving typically produces 10-20% higher CO2 than the WLTP figure due to variables like空调使用, passengers, luggage, driving style, and weather. This gap is acknowledged by the EU and UK governments and forms part of the argument for real-driving emissions (RDE) testing.

How to Reduce Your Vehicle is CO2 Footprint

  • Choose a hybrid or EV: Zero or very low CO2 emissions
  • Drive efficiently: Gentle acceleration and braking reduces fuel consumption and CO2
  • Reduce unnecessary weight: Remove roof racks and excess luggage
  • Maintain your vehicle: Well-maintained engines run more efficiently
  • Plan journeys: Combining trips reduces total miles and CO2

Conclusion

Understanding CO2 emissions UK: measured in g/km, determined by lab testing, affects VED directly. Lower CO2 = lower road tax. Use our car tax calculator to see your VED based on your CO2.