Your mileage may vary, as the saying goes.

But it turns out that real-world fuel efficiency in European cars varies considerably more from official ratings than in the U.S.

And some car brands are so far off from their ratings that a buyer may only get two-thirds of the published fuel efficiency.

DON'T MISS: Will European Fuel-Efficiency Tests Get More Realistic Under New Rules?

That's the conclusion of a new report, "Mind the Gap," published by Transport and Environment, a group based in Brussels that advocates for transportation policies to support sustainable development.

(The title is a reference to the phrase, familiar to every London Underground rider, that warns passengers to be careful of the gap between the car floor and the platform edge.)

Gas pump

Gas pump

As covered last week by trade journal Ward's Auto, the analysis said the gap between ratings and real-world results had widened from just 8 percent in 2001 to 31 percent for 2013, a discrepancy it called "staggering."

The results are so "distorted," T&E claims, that drivers likely see only about half of the improvement in efficiency promised by improved ratings since that time.

The report called out Mercedes-Benz in particular as having the largest gap between real-world fuel economy achieved on the road and efficiency ratings based on lab testing.

MORE: Hyundai, Kia To Pay $300 Million For Gas-Mileage Testing Errors

The problem, T&E says, is unrealistic test cycles in the standard New European Driving Cycle (NEDC) test used to measure emissions--and hence fuel efficiency--since 1996.

NEDC is “obsolete, unrepresentative of modern cars and driving styles," it claims, and hence it's "full of loopholes that car makers exploit to produce better test results.”

2015 Porsche Macan undergoes final testing

2015 Porsche Macan undergoes final testing

New global standard?

While a number of advocacy groups argue for a new and tougher test cycle, the ultimate solution may lie in the proposed Worldwide Harmonized Light Vehicles Test Procedure (WLTP).

This would be a global standard that all ratings agencies could use, meaning that for the first time, emissions and fuel efficiency could be directly compared for different vehicles sold in different countries.

ALSO SEE: EPA To Tighten Gas-Mileage Test Rules, Audit More Ratings From Carmakers

While the European Union would like that test introduced by 2017, the European automaker association--known as ACEA--wants it delayed until 2021 or later.

Meanwhile, in the U.S., the EPA said last month it would audit the ratings that carmakers submit far more closely,  as well as tightening and clarifying the rules for that testing.

BMW i3 electric car undergoing winter testing, February 2013

BMW i3 electric car undergoing winter testing, February 2013

In particular, the agency will better explain procedures to be followed for the so-called coastdown test, in which carmakers model and adjust for the effect of aerodynamic drag at varying road speeds on the cars they test on a stationary dynamometer, or "rolling road."

The U.S. moves follow widely publicized cases in which Hyundai, Kia, and Ford had to reduce fuel efficiency ratings for many models sold over the last few years, and send checks to owners of those car to reimburse them for the extra gasoline they had to buy.


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