CARS are using up to 25pc more fuel than manufacturers claim – and that is costing drivers €300-a-year more than figures suggest.
An extensive new study revealed carmakers were over-stating the mpg of their vehicles by a quarter on average, with some as high as 30pc.
And, according to the report, BMW, Audi and Mercedes have the biggest differences.
Although carmakers are not doing anything illegal, the report shows how current testing procedures allows them a lot of leeway.
'Official' tests are characterised by low acceleration, smooth road surfaces, super-efficient tyres, low weight allowances for driver/luggage, and constant cruising speeds – all significant contributors to low-fuel usage.
The 'consumption-gap' is one of the most contentious issues in the motor industry, with seller credibility under threat as the difference between 'headline' and on-the-road figures passes the €300-a-year mark.
The research, carried out by the non-profit International Council on Clean Transportation (ICCT), claims the gap is widening to 25pc from just 10pc a decade ago
It bases its verdict on the real-world returns from more than 500,000 private and company cars across Europe.
Luxury German vehicles showed the biggest divergence, the research suggests.
It claims BMW's emissions figures for its vehicles were, on average, 30pc lower than those found during conventional driving; Audi had the second widest disparity, with emissions around 28pc below actual use.
Mercedes showed a gap of 26pc, the authors said.
Figures for Toyota were much closer to real use – about 15pc less – while Renault and PSA (Peugeot-Citroen) were about 16pc lower.
Peter Mock, managing director of ICCT Europe, said: "This means that the actual fuel consumption experienced by the average driver is typically 25pc higher than what is printed on the sales sticker."
The results will also heap further pressure for an overhaul of EU testing procedures to make sure claimed fuel-efficiencies more closely reflect everyday use.
The switch to emissions-based car taxes is seen as having put extra pressure on carmakers to produce cars with lower consumption.
But the study's major criticism is that "tests do not take into account a number of conditions and behaviours typically found in on-road driving".
The consumption gap does have an upside for consumers who are driving cars that would, if stricter rules were applied, be paying more in purchase and road tax. However, it also means that governments are losing out in taxes.
The report adds that the new Worldwide Harmonised Light Vehicles Test Procedure, with its more realistic test cycle and tightened test procedure, will hopefully produce more realistic consumption figures.
For now the advice to buyers is to enjoy the lower emissions tax bands, take fuel consumption figures with a pinch of salt and drive more smoothly.