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News that Volkswagen overstated carbon dioxide efficiency sends share plunging

  • The exhaust pipes of an up to date Audi car blow out not visible emissions during the engine start in Gelsenkirchen, Germany, Wednesday, Nov. 4, 2015. Volkswagen said Tuesday that an internal investigation has revealed "unexplained inconsistencies" in the carbon dioxide emissions from 800,000 of its vehicles. (AP Photo/Martin Meissner)

    The exhaust pipes of an up to date Audi car blow out not visible emissions during the engine start in Gelsenkirchen, Germany, Wednesday, Nov. 4, 2015. Volkswagen said Tuesday that an internal investigation has revealed "unexplained inconsistencies" in the carbon dioxide emissions from 800,000 of its vehicles. (AP Photo/Martin Meissner)  (The Associated Press)

  • The instrument panel of an Audi car shows the fuel consumption in Gelsenkirchen, Germany, Wednesday, Nov. 4, 2015. Volkswagen said Tuesday that an internal investigation has revealed "unexplained inconsistencies" in the carbon dioxide emissions from 800,000 of its vehicles. (AP Photo/Martin Meissner)

    The instrument panel of an Audi car shows the fuel consumption in Gelsenkirchen, Germany, Wednesday, Nov. 4, 2015. Volkswagen said Tuesday that an internal investigation has revealed "unexplained inconsistencies" in the carbon dioxide emissions from 800,000 of its vehicles. (AP Photo/Martin Meissner)  (The Associated Press)

  • FILE - In this Sept. 22, 2015, file photo, the logo of Volkswagen at a car is photographed during the Car Show in Frankfurt, Germany. For years, Volkswagen lured customers with "clean diesel" ads that sent cars zooming off the lot and the company’s revenues spiraling. Now, in the wake of VW’s admission that its software cheated emissions tests, those ads could bring major sticker shock for the German automaker.  (AP Photo/Michael Probst, File)

    FILE - In this Sept. 22, 2015, file photo, the logo of Volkswagen at a car is photographed during the Car Show in Frankfurt, Germany. For years, Volkswagen lured customers with "clean diesel" ads that sent cars zooming off the lot and the company’s revenues spiraling. Now, in the wake of VW’s admission that its software cheated emissions tests, those ads could bring major sticker shock for the German automaker. (AP Photo/Michael Probst, File)  (The Associated Press)

Volkswagen shares are falling after the company said it had understated carbon dioxide emissions for 800,000 cars, adding to the downbeat news from its scandal over cheating on U.S. diesel emissions tests.

The company's ordinary shares slid 7.7 percent Wednesday to 102.40 euros in morning trading in Europe.

The carmaker said Tuesday after the European market close that it found "unexplained inconsistencies" in carbon dioxide emissions in some vehicles. The company found the additional problem as it investigated revelations that up to 11 million of its vehicles had software that allowed them to deceive emissions testers.

The announcement follows U.S. allegations that the defective software was also found in cars sold under Volkswagen's elite Porsche brand. CEO Matthias Mueller has promised the company will "relentlessly and completely clarify the matter."