(Bloomberg) -- Carmakers have to be clearer about the waythey certify their fuel-economy and emission ratings as regulatorsramp up scrutiny over the gap between laboratory results andon-road conditions, according to Daimler AG Chief Executive Officer Dieter Zetsche.

“You can only be transparent and if there’s any shortfallsanywhere, fix them and move forward,” Zetsche told Bloomberg Newsahead of the Beijing auto show opening this week. “And then ofcourse it will take some time” for the industry to be where theywere before the Frankfurt motor show last year.

Revelations that Volkswagen AG cheated on diesel emissionsemerged days after the Frankfurt exhibition last September. Sincethen, government fraud investigators have raided Frenchmanufacturer PSA Group as part of broader checks into vehicleemissions. In Japan, Mitsubishi Motors Corp. said last week itmanipulated fuel-economy tests to misleadconsumers.

Certification process

Also last week, Daimler said it had been asked by the U.S.Department of Justice to investigate the certification process ofits cars. The internal probe follows U.S. class action suits thatallege some of its cars violated emissions standards. Daimler hassaid it’s cooperating fully with authorities and that the suits are“baseless.”

The suits against Daimler say Mercedes-Benz clean-diesel modelscontain a device that causes the vehicles to violate U.S. emissionsstandards when run at cooler temperatures, making them lessenvironmentally friendly than advertised. Specifically, theautomaker was accused of using a device in its BlueTec cars to turnoff a system meant to reduce polluting nitrogen oxides in itsexhaust.

The U.S. EnvironmentalProtection Agency also contacted Daimler earlier this year overthe allegations, which Zetsche rejected in February as withoutmerit. While Daimler has

acknowledged the existence of a device, it says it’s legal anddesigned to protect the engine.

China regulation

“Will scrutiny increase? I would say probably. Yeah absolutely,”Mike Manley, who heads Fiat Chrysler Automobiles NV’s Jeepbrand, said at a briefing in Beijing. “The company has been fairlyclear in terms of performance against the standards regardless ofwhich market we are in.”

Going by the rules that are being proposed, China will be thetoughest regulatory regime over the next five years, FordMotor Co. Chief Executive Officer Mark Fields said on Saturday inBeijing, where the company showed the F-150 Raptor pickup truck.Public health and environmental concerns are driving the scrutiny,and the second-largest U.S. automaker would support China adoptingon-road emissions testing measures passed by the European Union, hesaid.

Related: VW said to pay at least $10 billion in U.S.cheating deal

China, home to the world’s biggest auto market with more than 24million vehicles sold last year, is aiming to improve fuel economyby about a third in 2020 from 2014 levels to reduce carbon-dioxideemissions, in large part by pushing automakers to introduceexhaust-free electric vehicles to balance against their sales ofgas-guzzling SUVs.

“It’s going to be tough,” said Hakan Samuelsson, Volvo CarsChief Executive Officer. “Legislation in China is world-class. Norelaxed regulations in China.”

Zetsche said that Daimler is “very active” in helping Europeanauthorities enhance and accelerate the definition of new testingcycles that give a close comparison between test and reality forfuel consumption.

Public understanding

“The same applies for the emissions,” he said at the sidelines ofan event on Sunday to unveil the stretched Mercedes-Benz E-class.“We think that is one important element that you get moresimilarity there and the general public can understand better thantoday because there is less difference between what you measureoutside and what you measure in the lab.”

Daimler shares fell 1.1% to 62.57 euros at 12:26 p.m. inFrankfurt, while Volkswagen dropped 1.6% to 123.45 euros.Mitsubishi Motors closed 4.8% lower at 480 yen on the Tokyoexchange. Ford declined 0.3% to $13.61 in U.S. trading onFriday.

While testing real-world driving emissions will give a morerealistic measurement, the move “will add to the technologicalcomplexity,” said Herbert Diess, who heads Volkswagen AG’s namesakebrand. “Treating emissions is very complex, and there are differentrules in many different countries. Today, emissions controls areabout as expensive as the engine itself.”

Related: VW's emissions retrofit may be among costliestrecalls ever

Even then, the numbers achieved at proving grounds and duringcertification will always differ from what consumers get inreal-world driving, according to Toyota Motor Corp.’s China chief,Hiroji Onishi. While the world’s largest automaker has engaged inno misconduct in meeting legal requirements, consumers will becarefully monitoring the difference, particularly with the hybridsit’s bringing to market promoted for their fuel efficiency, hesaid.

“We try to think on the actual usage of the vehicle and try towork in a way that will be helpful in the real world,” Onishi toldreporters on Sunday. “There will always be a gap between these twofigures.”

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