It's well known by now that the European Union testing protocol for fuel economy is much less rigorous than the U.S. Environmental Protection Agency (EPA) standards.

Both protocols are based around laboratory testing rather than real-world evaluation, but EU figures have gained a reputation for being much more optimistic than what drivers can realize in real-world use.

EU fuel-economy figures for a given model of car tend to be as much as 30 percent higher than their U.S. EPA counterparts.

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A new study demonstrates just how divorced from reality those ratings can be.

Conducted by environmental group Transport & Environment (T&E), it shows the industry-wide gap between laboratory and real-world fuel economy results has now reached 42 percent—compared to 28 percent three years ago.

The T&E study found that Mercedes-Benz was by far the worst offender, with some of its models returning real-world fuel economy more than 50 percent lower than official EU ratings.

2017 Mercedes-Benz E-Class (E300)

The average for Mercedes cars was 54 percent, while both the Mercedes-Benz A-Class and E-Class reached 56 percent.

Laboratory tests offer more controlled and less taxing conditions than real-world driving, which often leads to a discrepancy between lab results and what drivers can achieve in the real world.

But even taking those differences into account, a gap of more than 50 percent is still hard to explain, T&E suggests.

ALSO SEE: Peugeot admits real-world gas mileage is far lower than ratings

It "raises questions" about whether Mercedes used illegal "defeat device" software to cheat on tests, as the Volkswagen Group did in its TDI diesel models, the group said.

After Mercedes, Smart (owned by Daimler) and Audi tied for the second-largest gap, at 49 percent.

Many other automakers—including Peugeot, Toyota, and Volkswagen—also had gaps higher than 40 percent, while Fiat registered a gap of 35 percent.

2016 Mercedes-Benz A-Class

After conducting its own real-world fuel economy tests in concert with Transport & Environment this past fall, Peugeot admitted earlier this year that most of its models could not meet their rated fuel economy.

And it extended the admission from its own brand to the Citroën and DS brands it owns as well.

As a result of this disparity, European drivers spend an average of about 550 euros ($575) more per year on fuel than they would if the rated fuel-economy figures were applicable in the real world.

MORE: EU starts legal action against Germany, UK for failing to act on diesel emissions

Since news of the Volkswagen diesel scandal broke last fall, EU officials have debated using on-road testing for future fuel-economy ratings.

The EU has also begun legal action against the U.K., Germany, Spain, and Luxembourg for allegedly failing to introduce adequate penalties for automakers caught cheating on emissions tests.

The European Commission also publicized the fact that the Czech Republic, Greece, and Lithuania have no provision in their national regulations that would allow them to penalize against automakers that violate emissions rules.

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