By Barbara Lewis and Charlie Dunmore
BRUSSELS (Reuters) - The gap has widened between the fuel-efficiency that carmakers declare for their models and the reality for drivers, with luxury German vehicles showing the biggest divergence, a study has found.
Research by the non-profit International Council on Clean Transportation (ICCT) found "real-world" carbon emissions for new cars based on fuel consumption are about 25 percent higher on average than carmakers say, compared to 10 percent a decade ago.
The findings will add to pressure to reform vehicle testing procedures at international level and stoke ongoing EU debate on how to enforce 2020 car emissions goals for the 27-member bloc.
BMW reported vehicle emissions figures on average 30 percent lower than those found in actual use, said the report, published on Tuesday.
But BMW questioned whether the research was representative.
"The number of vehicles per carmaker that have been analyzed varies considerably and is based on only a very small and subsequently less representative section of our customer base," the carmaker said in a statement.
The ICCT, which aims to improve efficiency in transportation to benefit public health and mitigate climate change, said its report was based on data from nearly half a million private and company vehicles across Europe.
Volkswagen AG's luxury unit Audi had the second widest disparity, reporting emissions 28 percent below actual use, while Mercedes showed a gap of 26 percent.
Figures for emissions from Toyota vehicles were found to be about 15 percent less than in real use and Renault and PSA Peugeot Citroen's published data was about 16 percent lower than for vehicles on the road.
"This means that the actual fuel consumption experienced by the average driver is typically 25 percent higher than what is printed on the sales sticker," Peter Mock, managing director of ICCT Europe, said.
Previous research has shown how carmakers have perfected the art of lowering fuel use and resulting emissions in laboratory tests, through using tires with extra traction or unrealistically smooth driving surfaces.
Driving habits vary, meaning there will always be a discrepancy and exploiting loopholes is not illegal.
But the car industry agrees on the need for change. VDA, which represents the German industry, said in a statement it was working actively on reform of the testing regime.
The United Nations is leading a worldwide effort to update test procedures that date from the 1980s.
In parallel, the European Union is trying to tighten EU law on vehicle testing and to enforce a 2020 emissions goal of 95 grams of CO2 per kilometer (g/km) across the bloc.
Legislators in the European Parliament have said a tougher testing procedure should be introduced by 2017, but some EU member governments have been pushing for a delay until 2020.
The 95 g/km target for new cars from 2020 has been broadly agreed. Germany, however, has led calls for exceptions that campaigners say would seriously weaken enforcement of the goal.
(Additional reporting by Christiaan Hetzner in Frankfurt; editing by Rex Merrifield and Anthony Barker)