Some new EU cars, including Mercedes, BMW and Peugeot vehicles, were using around 50 percent more fuel than manufacturers claimed.
New European cars are spewing out on average 40 per cent more carbon dioxide than laboratory tests show, an environmental campaign group said on Monday, saying Volkswagen's rigging of emissions tests was only part of much wider cheating.
Volkswagen the world's biggest carmaker, has admitted using software known as a defeat device to ensure it passed U.S. testing for nitrogen oxides and said 11 million of its cars had been fitted with it.
The new report from Transport & Environment (T&E), which works closely with the European Commission, said its data did not prove other carmakers were using such devices.
But it said the gap between lab results and road performance had grown for emissions of carbon dioxide, as well as nitrogen oxides, to such an extent that further investigation was needed to discover what carmakers were doing to mask CO2 emissions.
Their analysis found some new EU cars, including Mercedes, BMW and Peugeot vehicles, were using around 50 percent more fuel than manufacturers claimed.
Fuel-use is associated with carbon dioxide production, so greater fuel use means higher emissions of the greenhouse gas.
T&E has worked for years to publicise the gap between lab results and real-world driving, producing an initial report in 1998.
In a statement on Monday, it said the gap was too wide to explain through well-known practices that have been tolerated in testing, such as taping up car doors to reduce wind resistance and using special driving surfaces and tyres.
The European Commission has proposed new legislation to narrow the gap and members of the European Parliament last week voted through an amendment to try to ensure it is approved on schedule and not diluted by lobbying.
T&E's analysis is based on work with the International Council on Clean Transportation, which helped to expose Volkswagen in the United States.
The analysis found the gap between official test results for CO2 and the real world rose to 40 percent on average in 2014 from 8 percent in 2001 for EU cars.
For some models it was higher. Mercedes cars had an average gap between test and real-world performance of 48 percent and their new A, C and E class models more than 50 percent.
"The Volkswagen scandal was just the tip of the iceberg," Greg Archer, clean vehicles manager at T&E, said, adding the CO2 gap costs a typical driver 450 euros ($504) per year.
Only Japan's Toyota would have met the industry's 2015 EU target of 130 grams of CO2 per kilometre (g/km) without exploiting test flexibilities, T&E said.
The target is tightened to 95 g/km by 2021 after carmakers were given an extra year to meet the new goal following an intervention by German Chancellor Angela Merkel in 2013.
ACEA, the European Automobile Manufacturers' Association, which represents top carmakers, has said there is no evidence the use of defeat devices, illegal in the EU, is an industry-wide issue.
In a statement on Monday it said the current test regime was developed in the 1980s and 1990s and therefore did not take account of new technologies that can affect fuel consumption. It added it supported the development of updated testing.
