Drivers have accepted for a long time that the mileage they get from their cars is never quite as good as the one claimed by the manufacturer. In Europe, at least, that’s because the rules allow automakers to test under idealized laboratory conditions that have no relation to actual real-world driving ones. But the International Council on Clean Transportation said in a new report Wednesday that the gap between reality and the industry’s claims is getting wider and wider.

After examining one million cars from across Europe, the ICCT concluded that the average discrepancy between the two had widened to 42%, from only 25% in 2013 and 15% ten years ago.

The ICCT relied on the Dutch-based Organization for Applied Scientific Research (TNO) and figures from consumer and driver associations in Germany, France, the U.K., Sweden and Switzerland.

Peter Mock, the ICCT’s managing director in Europe, said the gap was evidence that automakers “are exploiting loopholes in existing regulation more and more systematically.”

“For a third year in a row, virtually all the improvement in official car CO2 test results is hot air achieved by carmakers manipulating tests,” said Greg Archer, director for clean vehicles at the Transport & Environment watchdog in Brussels.

“The systems of testing and approving cars in Europe has been captured by the carmakers who abuse every loophole to circumvent the rules designed to prevent lethal air pollution emissions and climate change,” Archer said. “Until we have better tests and independent regulators cars will continue to pollute with impunity.”

The automakers argue that all the tricks of the trade–over-inflated tires, downhill-sloping test grounds and what-not–are all legal, as indeed they are. That’s thanks to the extensive lobbying of the European Union’s Commission and Parliament during the drafting stages of relevant law, documented in excruciating detail by non-governmental organisations like Corporate Observatory Europe.

Admittedly, the cheating wasn’t confined to European automakers. The ICCT’s sample included Japanese and U.S. cars, none of whom distinguished themselves particularly well. Indeed, Mitsubishi Motors lost its independence and its top management this year after coming clean on a decade-old culture of lying about mileage.

The ICCT noted that the gap costs the average driver in Europe around 450 euros ($480) a year in excess fuel costs. It also makes something of a mockery of the EU’s calculations of its contribution to global warming, given that road transport is responsible for almost a quarter of Europe’s greenhouse gas emissions.

But of most importance to the auto industry may be that fraudulent mileage claims are cheating EU treasuries out of millions in tax revenues. In Europe, most countries apply an annual tax to cars based on the amount of CO2 they are thought to emit.

“Since the divergence between real-world and type-approval CO2 emission values grew over time, governments incur increasing losses in tax revenue,” the ICCT said.

That means that the industry is offending a much more powerful and less indulgent subset of bureaucrats than the air quality police who were powerless to stop the diesel scandal.

UPDATE: This article has been updated to include comment from T&E.