Inertia of outdated regulation: How manufacturers can take advantage of India’s fuel economy test procedure


India introduced fuel consumption standards for passenger cars in 2017, and such regulations are among the most important policy tools for limiting vehicular CO2 emissions. To comply with the standards, manufacturers invested in a host of engine and vehicle technologies, and reducing vehicle weight is among the low hanging fruit, as a 10% weight reduction can easily give a 4%–5% improvement in CO2 emitted per kilometer. 

However, weight reduction can also be used to exploit a loophole in India’s current emissions testing procedure, and our analysis shows that in some cases, manufacturers are likely getting a higher benefit on the test than is achieved in real-world driving. As India is the fifth largest car manufacturer in the world, this is worth examining. 

The CO2 values used for fuel efficiency standards are measured in a test lab where environmental conditions are controlled and kept constant. A chassis dynamometer allows the vehicle to operate without actual movement on the road and replicates conditions as close as possible to those in the real world. The test procedure takes into consideration several forces that act upon a car in actual conditions, including vehicle inertia. To account for different types of vehicles ranging from smaller hatchbacks like Maruti Suzuki’s Alto to bigger SUVs like Toyota’s Fortuner, the test procedure defines a series of inertia classes. These classes are made up of weight categories in which the difference between the minimum and maximum weights typically varies from 60 kilograms (kg) to 230 kg. The equivalent weight of two persons, assumed as 150 kg, is added to the curb weight of the vehicle to determine its reference mass. That mass, in turn, is used to identify the inertia class in which the vehicle belongs. 

Every single vehicle whose reference mass falls between the minimum and maximum weight defined for an inertia class is assigned the same inertia on the dynamometer. This means that when a vehicle is run on the dynamometer, its actual vehicle weight may be greater or lesser, unless the reference mass of the vehicle is exactly the same as the inertia weight. An approximation of inertia forces is applied on the vehicle during a test on a dynamometer, with lower inertia forces leading to lower CO2 emissions. If manufacturers reduce weight just enough to put the vehicle at the upper end of an inertia class (i.e., just below an inertia weight class step), this provides an artificial advantage because the vehicle model will be portrayed by the test as more fuel efficient than it is. 

To what extent might manufacturers be seeking this advantage? To find out, we analyzed all new passenger cars sold in India in both fiscal year (FY) 2010–11 and FY 2018–19, and started with determining the inertia class of each vehicle. Then, the mass difference between minimum and maximum of each inertia class was divided into 10 equal bins. The bins were equally distributed within each of the inertia classes to allow us to understand the percentage distribution of vehicles in each bin and to understand how many vehicles fall just under the upper limit of a given inertia class. 

In FY 2010–11, India did not have fuel consumption standards and manufacturers voluntarily published the fuel consumption values obtained through the emissions test procedure. This means that there was no incentive for manufacturers to push for lower fuel consumption values other than having a marketing advantage. As shown in the brown bars in Figure 1, results of our analysis show that most vehicles are randomly distributed among bins further away from the inertia class steps. However, 14% of the vehicles were 0% to -10% below an inertia class step, and 2% fell in the bin 0 to 10% just above an inertia class step. This suggests that some manufacturers were optimizing vehicle weights to obtain a lower fuel consumption value. 

Figure 1. Distance from inertia step for all new passenger cars sold in India.

The same analysis for new cars sold in FY 2018–19 reveals that the percentage of vehicles just below an inertia class increased to 21%. The likelihood of a vehicle having a mass slightly below an inertia class step increased in the past 8 years, and that strongly suggests that manufacturers further optimized the weight of their vehicles with respect to the discrete inertia class steps. If no optimizing were taking place, a random type distribution would be expected. 

In Figure 2, below, we take a closer look at the fleet of vehicles made by Maruti Suzuki, a leading car manufacturer in India. Our analysis shows that the tendency to produce vehicles with weights that fall just below an inertia class step almost doubled to about 21% in FY 2018–19 from about 12% in FY 2010–11. 

Figure 2. Distance from inertia step for Maruti Suzuki’s fleet in India.

Vehicle segment also matters. If we look at the distribution of FY 2018–19 vehicles from all manufacturers whose weights are just below an inertia class, we find that 63% of them are compact hatchbacks and another 16% are sedans. This is because both these segments are very competitive in India and there is a much bigger incentive to market them as being highly fuel efficient. For a hatchback, a rough estimate is that a shift to one lower inertia class can give a 3.7% gCO2/km improvement. 

This all shows how India’s present inertia-class based test procedure can allow manufacturers to achieve lower CO2 values by simply minimally reducing weight to the extent that the vehicle falls in a lower inertia class. The reverse is also a concern, as manufacturers can also increase the weight of their vehicles without impacting the CO2 emissions in the lab results as long as the vehicle remains within the same inertia class. 

This test procedure is obsolete. Modern electronic dynamometers can simulate any vehicle weight by varying the load on the vehicle, and this avoids the technical limitation of hanging discrete inertia weights on the dynamometer. Thus, this method of gaming the inertia weight class system could easily be eliminated. 

A similar pattern as we see in India was observed in Europe 9 years ago, before the test procedure was switched to the Worldwide Harmonized Light Vehicles Test Procedure (WLTP). Along with the European Union, Japan and South Korea have also shifted to WLTP, which removes the inertia class step system and uses the exact vehicle weight for the inertia during a laboratory test. India should also adopt WLTP as soon as possible so that the gap between test and real-world fuel efficiency can be reduced.