ROUEN, France – European consumers will be in for a shock when Euro 5 emission rules arrive in 2014, accompanied by new testing procedures to replace the current New European Driving Cycle.

The new test is liable to show cars are 15% less efficient than thought.

An international effort to establish a Worldwide Light-Duty Test Procedure has petered out in the rest of the world, say auto makers at an engineering conference here on diesels, but European Commission regulators want to put it in place.

The NEDC and all other mileage homologation systems around the world specify acceleration, braking and gear-shifting details that allow one car’s mileage to be compared with another’s in a repeatable way.

However, real-world driving results differ from figures car makers show. Buyers rarely if ever attain the published mileage, because people don’t drive the way designated in the test cycle. In addition, auto makers tune cars to perform their best when following the national testing procedure, even if that means they do worse in real-world situations.

The International Council on Clean Transportation released a white paper in April citing figures from Germany showing drivers were getting fuel economy only about 8% worse than the auto makers’ figures in 2001, but today carbon-dioxide emissions are 21% more inefficient than published figures. In Europe, mileage is expressed by CO2 emissions per kilometer or by the number of liters of fuel needed to travel 100 km (62 miles).

European auto makers will face a huge marketing challenge when cars that now emit 100 g/km of CO2 suddenly are emitting 115 g/km.

Renault cars are about 15% less efficient in the real world than on the New European Driving Cycle, says Gasper Gascon-Abellan, vice president-powertrain planning, projects and partnerships.

“It will look like we are sliding backwards,” Gascon-Abellan says. The U.S., India and Japan have backed away from using a new test procedure and drive cycle, leaving Europe alone with confused customers, he adds.

But marketing is only one problem auto makers face.

“Today, our colleagues (in the EC) are playing with us,” says Christian Chapelle, vice president-powertrain and chassis at PSA Peugeot Citroen. “They are modifying the cycle, but we don’t know when. They say maybe January, maybe March, maybe next year. But at one time we have to launch the job, and the cycle, the level, everything is open.”

In general, a driving cycle reflecting real life would have more transitory moments and more changes in speed and acceleration. It would reflect a different state of charge in the battery, and whether air conditioning is on or off. It also would involve different ambient temperatures and test vehicles more closely would resemble the mass of the car being sold and its feature content.

Today, auto makers get their CO2 rating based on the simplest base vehicle, which has the least mass, and they tune the engine and transmission specifically for the NEDC.

The EC wants to make the new driving cycle the rule for when Euro 5 comes into play in 2014. But so far, says Andreas Shamel, managing director of the Ford research center in Aachen, Germany, “even the testing method is open.”

The regulators haven’t decided whether to use lab bench tests with random factors thrown in, or to have new cars fitted with equipment and tested while driving on roads.

“Depending on what method is chosen,” Shamel says, “it could mean potentially a complete revamp of testing facilities. It is a game going on there, and the lead times are really difficult.”

Improvements made in the past are real, admits the International Council on Clean Transportation. Average CO2 emission levels dropped from 158.7 g/km in 2007 to 140.3 g/km in 2011, according to the European Environmental Agency.

But because the improvements on the NEDC don’t reflect actual results on the highway, Europe isn’t saving as much fuel or limiting as much CO2 emissions as it wants to, the organization argues.