European car manufacturers are exploiting test loopholes to exaggerate their vehicles' green credentials, a study has found.
The report, which is likely to stoke already heated debate on carbon standards, found that cars are more polluting and a lot less fuel-efficient than their makers tell us.
Simulations used to test new cars have never perfectly reflected actual emissions.
However, the EC-commisioned analysis by three consulting firms found "flexibilities" squeezed consumers, benefited manufacturers and jeopardised European Union environment goals.
Test techniques such as using tyres with extra traction or driving on an unrealistically smooth road surface could account for about a third of the recorded drop in average carbon dioxide (CO2) emissions across the European Union between 2002 and 2010, it said.
"Frankly, people should be absolutely outraged. This is just taking money out of people's pockets. The industry is running rings around this procedure," one EU source told Reuters on condition of anonymity.
CO2 emissions were 167.2 grams per kilometre (g/km) in 2002 and 140.4 g/km by 2010, figures in the report showed, giving a total average reduction across new EU cars of 26.8 g/km.
The study attributed 9.1 g/km, or roughly a third, to the way testing was performed, rather than improved technology.
"This means that vehicles do not deliver end-users the promised fuel cost reductions, leading to consumer misinformation," said the report carried out by the Netherlands Organisation for Applied Scientific Research (TNO), British-based AEA Ricardo and IHS Global Insight, of the United States.
Already widely used, the flexibilities could be exploited further as debate continues in Brussels on implementation of a 2020 target to cut average emissions across the EU fleet to 95 (g/km).
In addition to the 2020 goal, the Commission is revising testing law, but it is not expected to close all the loopholes. Globally, the United Nations is working on new standards.
The Commission said new tests from around 2016 should "mitigate" the effect of these flexibilities on the gap between actual and regulatory CO2 emissions, though "some tolerances are necessary for practical reasons".
EU consumer organisation BEUC calculated that the flexibilities meant that consumers paid up to 135 euros (£114) a year more in fuel, based on today's fuel prices and 14,000km of driving in a car bought in 2010.
British Liberal member of the European Parliament Chris Davies said he was working on amendments to tackle testing standards as part of the 2020 cars emissions debate.
"The cheats are confounding the lawmakers and deceiving the public," he said.
Another British Liberal European politician, Fiona Hall, is calling for conformity tests after cars have entered service.
One of those involved in the report, TNO consultant Richard Smokers, said that such tests would help and that Europe's use of flexibilities was more pronounced than elsewhere.
The United States already has in-service tests and Japan is culturally scrupulous, he said.
"What we have heard from people in the field is that there is a cultural reluctance to exploit flexibilities," he said.
"It's the difference between the spirit and the letter of the law.
"In Europe, we have a tradition of finding and exploiting bandwidths and loopholes."
VDA, which represents the German auto industry, said in a statement that a unified testing cycle was necessary and the car industry was "working actively" on reform.