Official fuel consumption measured in tests is on average almost a quarter lower than that achieved by drivers on the road.
The report finds that this gap is growing and the principal cause is car manufacturers manipulating official tests.
The report, published today by sustainable transport campaigners Transport & Environment (T&E), explained why the gap between real-world CO2 emissions figures and test results actually occurs.
It includes a comparison of test results produced by carmakers and identical tests performed by an independent laboratory on six standard new cars.
The results show that the CO2 and fuel consumption figures achieved in official test results (that use specifically prepared cars) are on average 23 per cent lower than independent tests using regular production cars that do not seek to exploit flexibilities in the testing procedures.
These results match the difference between typical test results and real-world averages.
According to the report, there are about 20 ways carmakers ‘creatively’ re-interpret test procedures, exploiting loopholes and flexibilities in the current rules to boost the green credentials of their cars.
Among the tricks applied are: taping over cracks around the doors and grills; over-inflated tyres; adjusting the wheel alignment and brakes; using special super-lubricants; minimising the weight of the vehicle; testing at altitude, at unrealistically high temperatures and on super-slick test-tracks.
The report concludes that the current test regime is not fit for purpose and that loopholes in the current test should be immediately closed, with new cars and vans retested under realistic conditions.
Greg Archer, clean vehicles manager at T&E, said: “This new evidence shows that carmakers in Europe are cheating their own customers by manipulating official tests, which leads to thousands of euros of additional fuel costs for drivers.
"They are also cheating legislators, as EU laws intended to reduce CO2 emissions from cars and vans are only being met in the laboratory, but not on the road.
"The only way to rebuild this trust is by closing loopholes in the current test procedures, to ensure that cheaters never prosper.”
From 2016, T&E is calling for a new test cycle, the World Light Duty Test Procedure (WLTP), to be introduced and for checks to be made on new production cars to ensure they can replicate test results.
“Average emissions of 95g CO2/km in the regulation should deliver 95g on the road,” Archer said..
"Loopholes in the ways cars are tested and flexibilities such as double counting of electric vehicles using so-called ‘supercredits’ just weaken the intended benefits of the regulation and are not needed."
Download the full report