18 Nov 2014

(Sur)realism in measuring the fuel economy figures of cars

Felix B writes:*

EU rules on carbon emissions require car manufactures to reduce their average car to a maximum of 130 grams of CO2 per kilometre by 2015, and 95 grams per kilometre by 2021. The scheme that is used to measure the fuel economy figures – the New European Driving Cycle (NEDC) – aims to represent the typical fuel use of a car and is used by the EU for its rules introduced above. However, car buyers as well as car magazines have stated (pdf) that the fuel use measured by the NEDC is drastically lower than the real world fuel use of cars. Thus the question rises what causes NEDC values to vary from real world fuel consumption figures.

First, the testing cycle is performed on a roller test bench. The absence of real road testing affects the measurement of fuel economy, as roller test benches cause less resistance on the tyres, which ultimately decreases fuel consumption. Furthermore, roller test benches do not account for differences in weather or traffic conditions. For example, the test's speed pattern is signified with low accelerations and numerous constant speeds cruises (i.e. from 70 km/h to 100 km/h in 35 seconds), but real traffic conditions are much more abrupt and dynamic, whilst constant speed cruises can usually be found only on motorways. Dynamic and numerous accelerations (i.e. in stop/go traffic) as well as rapidly changing speeds significantly increase fuel consumption. Thus, the current test's laboratory nature severely discredits its real world applicability and allows car manufacturers to artificially smarten up the fuel consumption of their cars.

Moreover, the measurement is conducted with all supplementary systems such as air conditioning, heated rear window, radio, and light turned off. Whereas this increases comparability across vehicles, the absence of any supplementary system is not representative of the real world – sometimes these are even mandatory (i.e. light during the night or in bad weather conditions). The dismissal of supplementary systems itself already reduces fuel the measured fuel consumption. What aggregates this effect is that supplementary systems increase weight, which in turn increases fuel consumption. Car manufacturers use cars with the lowest possible number of supplementary systems or even take them out of the car before the test to reduce the weight of the tested car. Therefore, the measured fuel consumption is unrealistic of the real weight of a car and its use of ancillary systems.

The surrealism of the testing procedure allows car manufactures to cheat the EU laws designed to improve fuel efficiency and reduce emissions. The costs will be borne by consumers of cars and the environment. Drivers will need to pay more for fuel than they initially assumed based on the official fuel rates of their cars. The cumulative additional CO2 emissions will accelerate and intensify climate change. The oil required for the fuel will have to be imported and will alter the balance of payments and affect growth, since money will flow out of the EU economy. Because any current emission or tax policy is based on the unrealistic fuel efficiency values of the NEDC, the polices do not correctly account for the negative spill overs caused by cars in the real world. Thus, any such pollution tax on cars or car manufacturers is too cheap as compared to the real pollution caused by cars.

Bottom Line: Whereas the NEDC allows for comparability across cars (as conditions are always the same), it simply allows car manufactures to cheat with and under comparable conditions. Despite cheating consumers, the surrealism of the test leads to an understatement of costs of cars to the environment and results in wrong policies and too low taxation schemes.

* Please comment on these posts from my environmental economics students, to help them with unclear analysis, other perspectives, data sources, etc.


  1. Felix, you seem to have missed the discussion, typically in beginning physics classes, about accuracy versus precision. The most common analogy used for illustrating accuracy and precision is shooting at a target. If I fire a gun aimed at the center of a target several times and all of the bullet holes are tightly clumped 6 inches below and 4 inches to the right of the center of the target then the gun is inaccurate but very precise. If I want to hit the center of the target I simply aim 6 inches too high and 4 inches to the left. The EU standard does not have any need for an accurate measure of carbon emissions in real world conditions if the test has high precision and you know something about the inaccuracy. Since you and the car magazines have been able to so readily see the inaccuracies in the testing procedure, I'm pretty sure the EU and the organizations that established the testing procedure also have a pretty good handle on the accuracy of the testing procedure too. I'm also pretty sure that the standardization of the test makes it quite precise. I also expect that it is well understood, by those establishing the test, how manufactures try to "game the system" and it is very likely accounted for if necessary.

  2. I agree with Brian.

    I will add one caveat. The consumer may use fuel consumption figures determined from the use of government sponsored (required) tests with little skepticism. They will likely have a false sense of confidence because they may believe (incorrectly) that the government standard was created to help them.

    The government mandated test may still allow them to rank order different vehicles, but if they want realistic fuel consumption figures they may have to rely on market driven sources such as car magazines or Consumers Reports.

    Bottom Line: Let the buyer beware - even if they are using government mandated information.


Read this first!

Make sure you copy your comment before submitting because sometimes the system will malfunction and you will lose your comment.

Spam will be deleted.

Comments on older posts must be approved (do not submit twice).

If you're having problems posting, email your comment to me