The AIR Index CO2 Ratings

The AIR Index urban NOx ratings were launched in February 2019 based on test data collected in accordance with the CWA17379 methodology developed for cars and light-duty vehicles (vans).

The AIR Index urban NOx ratings were launched in February 2019 based on test data collected in accordance with the CWA17379 methodology developed for cars and light-duty vehicles (vans). The AIR Index ratings provide trusted, independent and comparable insight to inform vehicle choice and policy decisions based on the actual NOx emissions produced during urban driving.

In consultation with AIR’s Scientific Advisory Committee (SAC), AIR will now test the tailpipe CO2 emissions from cars and light-duty vehicles (M1 and N1 categories) during on-road and extra-urban driving. The results for each vehicle will be published as the AIR Index CO2 ratings in bands from ‘A’ the lowest to ‘E’ the highest to provide trusted, independent and comparable insight to inform vehicle choice and policy decisions to reduce the vehicle related carbon footprint.

This document describes the two elements required for the testing of vehicles and publication the AIR Index CO2 ratings:

  • The testing methodology for collecting tailpipe CO2 emissions which follows the established principles developed for the AIR Index NOx ratings conducted according to the CWA 17379 standardised method but with addition of extra-urban driving for all and two load states for light-duty vehicles.
  • The banding and description for the AIR Index CO2 ratings, including the fuel economy and consumption recorded during the testing programme.

Test Methodology for the AIR Index CO2 ratings

Testing vehicles to collect the data required to produce ratings for tailpipe CO2 emissions must include both urban and extra-urban driving.

Urban Driving

AIR established the principles of collecting comparable vehicle emissions data for urban driving using the CWA17379 methodology following the CEN Workshop agreement protocols and process. This methodology requires at least two matching vehicles to be tested, to collect data during a minimum of three compliant tests and five valid trips, within the specified boundary conditions. The tailpipe CO2 emissions for urban driving are collected according to the exactly the same protocol for NOx emissions for urban driving in CWA17379.

Extra-urban Driving

The data for extra-urban driving are collected by following the same methodology as per CWA17379, but with the necessary variations in dynamic boundary conditions including speed, acceleration, altitude and acceleration to reflect the differences in journey type between city and motorway driving. AIR has analysed all extra-urban data over eight years of testing conducted by Emissions Analytics and used the distribution of those testing dynamics to inform the choice for the AIR Index boundary conditions.

Collection of on-road, extra-urban data also requires at least two matching vehicles to be tested. Data are collected during a minimum of three compliant tests from five valid trips, within the specified extra-urban boundary conditions.

The chart below is an extract of Table 6 from CWA17379 listing the urban boundary conditions, along with additional columns showing the boundary conditions for extra urban driving.

Boundary Condition Unit Urban Data Extra-Urban Data
Min Value (Inclusive) Max Value (Inclusive) Min Value (Inclusive) Max Value (Inclusive)
Avarage Speed km/h 20.0 40.0 95.0 115.0
Relative Positive Acceleration m/s2 = -0.0016v+0.1755 = -0.0016v+0.250 = -0.0016v+0.1755 = -0.0016v+0.250
V*_positive @95th percentile m2/s3 = -0.03v+7.0 = -0.05v+12.0 10 25
V*_negative @95th percentile m2/s3 = -0.03v+7.0 = -0.05v+12.0 -35 -10
Cumulative positive altitude gain m/100km 0 600 0 2,000
Altitude difference between start and end M -60 60 -200 200
Average altitude M 0 700 0 700
Average ambient temperature Degrees Celsius (°C) 10 20 10 20
Mass above Mass in running order Kg 140 260 140 260
Vehicle mileage Km 3,000 100,000 3,000 100,000

All other boundary conditions and test requirements are as per the CWA17379 standard methodology.

In addition, for both urban and extra-urban trips there is an important additional requirement. As driver behaviour and vehicle conditioning can potentially have a significant effect on the result, even where all the boundary conditions are met, it is important that all tests are conducted by drivers that have been appropriately trained. The drivers must be validated by external agencies ensuring that the driving style remains similar over time and between drivers.

In line with CWA17379, AIR has inspection rights for extra-urban data collection over any test vehicle to ensure no modifications have been made from the default production condition.

In addition to the collection of tailpipe CO2 during the urban and extra-urban driving, the fuel economy will be recorded in miles per gallon (Mpg) and converted to consumption expressed in litres per 100km, and both measures will be published.

Banding and description for the AIR Index CO2 ratings

The tailpipe CO2 data collected according to the methodology described above will be presented as a single number in g/km for each vehicle and banded in AIR Index ratings from ‘A’, the lowest to ‘E’, the highest. These will be known as the AIR Index CO2 ratings.

The AIR Index CO2 ratings are created by combining the tailpipe CO2 emissions data from urban and extra-urban driving by applying the same 52/48 weighting by distance as per the WLTP regulations. WLTP was developed with the aim of being used as a global in-laboratory test cycle across different world regions, so pollutant and CO2 emissions as well as fuel consumption values would be comparable worldwide.

AIR also considered weighting according to the former NEDC regulations (50/50) and US Environmental Protection Agency (55/45) from the USA but rejected them both in favour of the WLTP global perspective. The AIR Index CO2 rating bands and descriptions are shown below.

Combined CO2 Emissions Range AIR Index CO2 Rating What does the rating mean
0 – 95 g/km
(EU average target 2021 for cars is 95 g/km)
Low levels of CO2 emissions
95 – 130 g/km
(EU average target 2021 for cars is 130 g/km)
Moderate levels of CO2 emissions
130 – 180 g/km
(EU average target 2021 for cars is 147 g/km)
High levels of CO2 emissions
180 – 230 g/km Very hight levels of CO2 emissions
230+ g/km Extreme levels of CO2 emissions

The fuel economy in Mpg (and consumption in litres per 100km) recorded during the urban and extra-urban test will also be displayed in the results page for the vehicle on the AIR Index website.