The gap between official and real-world fuel-economy figures in Europe has risen to about 38%, according to a new report published by the International Council on Clean Transportation (ICCT).

Ten years ago the discrepancy between these real-world and sales-brochure values was at 10%. The new report—“From Laboratory to Road: 2014 Update”—updates ICCT’s original 2013 report on the growing discrepancy.

The new report is based on data from more than half a million private and company vehicles across Europe. The findings come as the European Commission is preparing to adopt an improved test procedure that would produce more realistic vehicle test results.

The new report, jointly prepared by the ICCT, the Netherlands’ Organisation for Applied Scientific Research (TNO), and Germany’s Institut für Energie- und Umweltforschung Heidelberg (IFEU), unveils the increasing real-world efficiency gap using systematic statistical analysis.

A technically precise definition of real-world driving conditions is elusive because of the large variation in vehicle types and driving behavior. However, by aggregating large sets of on-road fuel consumption data, clear trends can be observed. This methodology is the basis for the analysis in this report. The data sources comprise eight different data sets covering as many as 13 model years, including both private and company cars, from various European countries. In total, these sources furnish fuel consumption and CO₂ emission data from more than half a million vehicles.

The data show that for private cars, the difference between on-road and official CO₂ values rose from around 8% in 2001 to 31% in 2013. For company cars, the gap is even greater: 45% in 2013.

The analysis draws on data from a number of different sources: the user websites spritmonitor.de (Germany) and honestjohn.co.uk (United Kingdom); the leasing companies Travelcard (Netherlands) and LeasePlan (Germany); the car and consumer magazines AUTO BILD (Germany), auto motor sport (Germany), WhatCar? (United Kingdom); and the car club TCS (Switzerland).

As one result of the growing spread, motorists are spending an average of €450 (US$570) per year more on fuel than if that non-performance gap were closed. The increasing gap also more than halves the official CO₂ reductions achieved during the last ten years, making it more challenging to meet the EU’s climate change mitigation objectives. For governments, the growing gap causes significant losses of tax revenues. This is because vehicle taxes are based on laboratory test results instead of real-world data. For the Netherlands alone, the loss in tax revenues could exceed €3.4 billion (US$4.3 billion) per year, according to the ICCT estimate.

One new feature in the 2014 report is an examination of vehicle models. When looking at individual vehicle models, the ICCT researchers found that when a new vehicle model generation is introduced, the gap suddenly increases from one year to another Looking at 26 of the top-selling models in the EU, they found that for vehicle model changes before 2009, each time a new generation came on the market, the average gap increased from 12% to 17% of the official CO₂ value from one year to the next.

For more recent model changes—after the EU CO₂ regulation for new cars was introduced in 2009—they found that the typical increase in the gap from one generation to the next jumped even more, from 18% to 29%.

The authors noted that “it is reasonable to assume that driving behavior has not changed appreciably over the past years.” Instead, they suggested that the observed increase of the gap is most likely due to a combination of:

• Increasing application of fuel-saving technologies that show a higher benefit in type-approval tests than under real-world driving conditions (for example, stop- start technology).
• Increasing exploitation of “flexibilities” (test tolerances and insufficiently defined aspects of the test procedure) in the type-approval procedure (for example, during coast-down testing).
• External factors changing over time (for example, increased use of air conditioning).

Manufacturers measure vehicle fuel consumption in a controlled laboratory environment, using a test procedure called the New European Driving Cycle (NEDC). This procedure was developed in the 1980s and was not originally intended to be used for fuel consumption testing but air pollutants. A new and more appropriate test procedure, the Worldwide Harmonized Light Vehicles Test Procedure (WLTP), has been developed through the United Nations and is ready for implementation in the EU as early as 2017.

In the new test procedure, for example, the weight of the vehicles will be reflected more realistically—one of the aspects that will help to reduce the current fuel-economy gap.

It is important to clarify that this analysis is not intended to incriminate vehicle manufacturers. The NEDC was not originally designed to measure average fuel consumption or CO₂ emissions, and some of its features can be exploited to obtain artificially low results. Manufacturers appear to be taking advantage of permitted flexibilities in the NEDC, resulting in lower CO₂ emission values. The new Worldwide Harmonized Light Vehicles Test Procedure (WLTP), with its more dynamic test cycle and tightened test procedure, is expected to result in somewhat more realistic values. … A key aspect of the transition will be an appropriate conversion of existing CO₂ targets and CO₂ based taxation schemes from NEDC into WLTP. Unintended flexibilities that are currently part of the NEDC should not be accounted for when transitioning to WLTP. Otherwise there is a risk of undermining the introduction of WLTP and making most improvements achieved with the WLTP obsolete.

At the same time, the WLTP will not resolve all open issues, and the new procedure may itself have vulnerabilities that have not yet been identified. In light of this, it is important to complement the WLTP with additional measures, such as testing and regulating the efficiency of vehicle air conditioning systems. More importantly, some form of in-service conformity testing should be required to ensure that reasonable emission values are achieved not for a single test vehicle alone but for all cars sold to customers and driven on the road.