Berlin - Vehicles on European roads pump out far more carbon dioxide (CO2) than advertised, the team of scientists who uncovered Volkswagen's diesel emissions scandal reported in a study released on Thursday.

The study by the International Council on Clean Transportation (ICCT) looked at data from about 1 million cars in six countries and found that the gap between CO2 levels promised and that actually produced was about 42 percent.

In 2001, the gap was reportedly only 9 percent.

That means customers concerned about environmental pollution could be buying cars that potentially pump out more carbon dioxide than they expect. Car owners could also be paying up to 450 euros (R6880) a year more in fuel costs than expected based on fuel efficiency promises.

“About three-quarters of the gap between laboratory test results and real-world driving is explained by vehicle manufacturers exploiting loopholes in the current regulation,” said ICCT Europe managing director Peter Mock.

The group recommends internationally standardised testing standards that would force carmakers to test vehicles in real world conditions.

The group noted that carmakers do things like make sure batteries and tyres are in perfect condition before tests, not always the case for everyday vehicles. Others exploit technology in the laboratory that reduces emissions, which would be impossible to employ during standard use.

The ICCT made headlines last year when its tests of Volkswagen's diesel cars found the company had employed technology to spoof emissions results, making cars seem greener than they were.

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