How carmakers overly optimise their fuel consumption figures ?
First is determined the rolling resistance ⓘ of the test vehicle by driving it on an outdoor track. This resistance is mainly linked to aerodynamics and weight.
CO2 emissions of the car are then measured on a test bench in a laboratory with a standardised driving behaviour (accelerations, speed, breaking). The rolling resistance previously calculated is taken into account during this lab test.
The gap between official and real fuel consumption is due to a fuel consumption optimisation that is not representative of real driving conditions.
To reduce the rolling resistance, carmakers over inflate the tires, seal doors and radiator grilles with tape, take off exterior mirrors, drive the car downhill and can even remove the doors to lighten the car.
In the lab they can switch off air conditioning and lights, as well as interrupt battery charging via the alternator.
On top of that some manufacturers use softwares (also called defeat devices ⓘ) that identify the testing conditions and optimise the results during the test.
For example, a software discovered by the California environmental agency (CARB) lowers the CO2 emissions of the car as long as the steering wheel is not moved, as on a test bench. As soon as the steering wheel is turned by more than 15 degrees, another program is activated and the vehicle consumes more fuel.
Deutsche Umwelthilfe e.V. (DUH, Environmental Action Germany) and Transport & Environment (T&E) have been pointing out the discrepancy between official and real values for years. DUH first disclosed this fraud and explained in detail how it came about in 2007.
What does the law say?
Car manufacturers are obliged to issue honest statements about real-world fuel consumption figures. The corresponding EU regulation stipulates that the emissions measured in the type-approval ⓘ test must correspond to those emitted in real driving conditions. These values must be maintained throughout the normal life of the vehicle under normal conditions of use.