In order to be placed on the market, passenger cars carry out a series of tests to verify their compliance with regulations. The tests to assess fuel consumption, CO₂ and pollutant emissions are carried out in the laboratory and are based on specific driving cycles. In this way, the tests are reproducible and the results comparable. This is important because only a laboratory test, which follows a standardised and repeatable procedure, allows consumers to compare different car models.

On 1 September 2017, the new Worldwide harmonised Light-duty vehicles Test Procedure (WLTP) came into force and will gradually replace the New European Driving Cycle (NEDC) protocol.

The new WLTP procedure is characterised by a more dynamic driving profile with more significant acceleration. The maximum speed increases from 120 to 131.3 km/h, the average speed is 46.5 km/h and the total cycle time is 30 minutes, 10 minutes more than the previous NEDC. The distance travelled doubles from 11 to 23.25 kilometers. The WLTP test consists of four parts depending on the maximum speed: Low (up to 56.5 km/h), Medium (up to 76.6 km/h), High (up to 97.4 km/h), Extra-high (up to 131.3 km/h). These parts of the cycle simulate urban and suburban driving and driving on extra-urban roads and motorways. The procedure also takes into account all vehicles' optional contents that affect aerodynamics, rolling resistance and vehicle mass, resulting in a CO₂ value that reflects the characteristics of the single vehicle.

The WLTP procedure will gradually replace the NEDC procedure. The WLTP applies to new passenger car models from 1 September 2017, to all passenger cars registered from 1 September 2018 and is mandatory for all EU Member States.

Until the end of 2020, both fuel consumption and CO₂ emission values in WLTP and NEDC will be present in the vehicle documents. Indeed, NEDC values will be used to assess the average emissions of cars registered in the EU throughout 2020. In addition, some countries may continue to use the NEDC data for fiscal purposes. From 2021 onwards, WLTP data will be the only consumption/ CO₂ emission values for all cars. Used vehicles will not be affected by this step and will maintain their certified NEDC values.

The new WLTP test procedure is more representative of current driving conditions than the NEDC procedure, but it cannot take into account all possible cases including the effect of the driving style that is specific to each individual driver.

Therefore, there will still be a difference between emissions and consumption measured in the laboratory and those resulting from the use of the vehicle in the real world, and the extent of this difference will depend on factors such as driving behaviour, the use of on-board systems (e. g. air conditioning), traffic and weather conditions that are characteristics of each geographical area and each driver.

For this reason, only a standardised laboratory test allows to obtain values with which it is possible to compare vehicles and different models in a fair way.

The RDE test verifies that cars maintain low emissions even under real road driving conditions. The RDE test does not replace but complements the WLTP laboratory test. During the RDE test, the car is driven on the road in a wide range of different conditions. Conditions include different altitudes and temperatures, additional payload, uphill and downhill driving, urban roads (low speed), rural roads (average speed), highways (high speed).

In order to measure pollutant emissions while driving the vehicle on the road, test cars are equipped with portable measurement systems (PEMS) that provide full real-time monitoring of the main pollutants emitted by the vehicle. PEMS are complex devices that integrate advanced gas analysers, exhaust gas mass flow meters, weather station, satellite-based geolocation and a network connection. The data collected is analysed to verify that the boundary conditions of the RDE journey have been met and that emissions are within the limits of the regulation.

The limits not to be exceeded are defined as those prescribed in the laboratory test (WLTP) multiplied by compliance factors. Compliance factors shall take into account the margin of error of the instrumentation that does not measure at the same level of accuracy and repeatability as that used in the laboratory test.

The RDE test shall apply from 1 September 2017 for new passenger car models, and to all passenger cars registered from 1 September 2019 in the case of NOx, with a conformity factor of 2,1. On the other hand, in the case of particle number (PN) verification, the compliance factor is 1.5 and applies to all passenger cars registered as from 1 September 2018.