With each test you will find a score for CO2, Local Air Quality and overall. These scores give an indication of how the test vehicle compares to the average diesel vehicle. The average diesel vehicle is the average emissions of all of the new diesel vehicles on sale in the UK. A positive score indicates that the vehicle has higher emissions than the average diesel whilst a negative score indicates that the vehicle has lower emissions. All of the emissions data is taken from the Vehicle Certification Agency website.

CO2 is the main greenhouse gas emitted by vehicles making up around 80% of the greenhouses gases emitted by each vehicle. Purchasing a vehicle with lower CO2 emissions will save you money, as fuel consumption and CO2 emissions are linked and both road tax and company car tax are calculated based on CO2 emissions.

It is worth noting that NOx are also global warming gases emitted by vehicles. The global warming potential of a gas (GWP) is expressed in relation to the GWP of CO2 which has a GWP of 1. A gas with a GWP of 2 is twice as powerful, a gas with a GWP of 3 is three times more powerful and so on.

NOx from vehicles is made up of around 95% NO and 5% NO2, however the NO quickly reacts with oxygen in the air to form NO2. NO2 has a GWP of 7.

When choosing a vehicle it is important to take into account both the global warming and local air quality impact. The overall mark provides a quick way of accessing this as both CO2 and the emissions that lead to poor local air quality are given equal weighting.

If you have a particular interest in either CO2 emissions or the emissions that lead to poor local air quality you can check the scores individually.

Which gases contribute to poor local air quality?
The local air quality score takes into account the following regulated emissions, Carbon Monoxide (CO), Hydrocarbons (HC), Nitrogen Oxides (NOx) and Particulate Matter (PM). To find out more about the effects of these gases please visit the Pollution section of this site.