An environmental research group made a study revealing the reasons why there are such big discrepancies between real-world emissions and fuel levels and official ones.
On average, real-world fuel consumption and CO2 emissions of new European passenger cars exceeded official vehicle type-approval values by around 40 percent in 2014, according to the International Council on Clean Transportation. A study made by the group shows that about one-third of this gap can be explained by automakers systematically exploiting “technical tolerances and imprecise definitions” in type-approval testing rules. The study analyzed real road-load data for 29 passenger cars from model years 2009–2012 provided by four independent vehicle test labs. For 19 of the 29, it compared the real-world road loads to official road loads from the French and German type-approval agencies.
The road-load tests are the ones that measure forces of inertia, friction, and aerodynamic resistance affecting a vehicle on the road. The tests, during which a vehicle is driven and then coasted, determine parameters required to set up a chassis dynamometer to simulate road load during a type-approval test.
According to the researchers, the road-load coefficients greatly influence official fuel consumption and CO2 emissions values, because energy is required to overcome road load. If the coefficients used to set up the type-approval test simulate too little road load, the car will consume an artificially low volume of fuel for the test, and emit less CO2.
Actual road load exceeded what was simulated during type-approval tests on those vehicles. The study estimates that the influence of unrealistic road load data on CO2 emissions and fuel consumption tests produced official values between 0.7 percent and 14.5 percent lower than they would have been if actual real-world road load data had been used.
Official CO2 emission figures were on average 7.2 percent lower than real ones because of the inaccurate official road-load coefficients, ICCT said. That figure explains one-third of the average gap between official and real-world CO2 values for new cars.
ICCT also analyzed and compared data from those models sold in the United States and the average impact on CO2 emissions and fuel consumption of differences between the official US road-load coefficients and the real-world data was only 1.8 percent.