The environmental body said some carmakers’ claims are unachievable and it further accused the industry of “manipulating” testing procedures.
An Taisce called on the EU to reform the car testing system to avoid the publication of “delusional” fuel economy figures.
However, the body, otherwise known as the National Trust for Ireland, conceded the motor industry was not breaking any rules.
It pointed out a new study had shown fuel consumption and emission levels in new cars were 23% above claims by carmakers.
An Taisce said such claims could be made because of current loopholes in the EU’s procedures for testing.
“It’s an easy system to manipulate because manufacturers do things during the fuel cycle test that drivers would never do,” said An Taisce spokesperson James Nix.
Among the measures used by car testers to achieve good fuel consumption and emission figures, according to An Taisce, are over-inflating tyres, taping gaps between doors, testing at altitude and removing passenger side mirrors. Such practices were highlighted in a report published yesterday by an EU-wide federation, Transport & Environment.
An Taisce said loopholes in the testing system set down by the EU allowed car manufacturers to massage fuel economy results without being in formal breach of any rules.
Mr Nix said consumers would regard the EU’s testing system with disillusionment unless it moved quickly to revise the system.
However, the Society of the Irish Motor Industry said the existing testing system, in use for 20 years, had demonstrated major improvements in fuel efficiency and lower carbon emission levels in that period.
SIMI director general Alan Nolan said the system, under review by the EU, allowed for comparisons between different models based on standardised tests.
Mr Nolan acknowledged it wasn’t always possible to replicate everyday driving in test situations but said test results remained valid.