Volkswagen 2015 car models are shown for sale at New Century Volkswagen dealership in Glendale, Calif., on Monday, Sept. 21, 2015. AP Photo/Damian Dovarganes FRANKFURT (Reuters) - Investors wiped another 4 billion euros ($4.4 billion) off Volkswagen's market value after fresh admissions from the carmaker threatened to make a serious dent in car sales for the first time since a test-cheating scandal erupted.

VW said late on Tuesday it had overstated claims about the fuel efficiency of 800,000 cars in Europe, and analysts in response said it could potentially hurt those cost-conscious consumers who have so far taken VW's manipulation of smog-causing emission tests in their stride.

The carmaker said carbon-dioxide emissions had also been understated and added 2 billion euros to its estimated costs of the scandal, bringing a new dimension to an issue thus far thought to affect the nitrogen-oxide emissions of 11 million diesel vehicles.

"Another week, another shock in the VW story," Exane BNP Paribas analyst Stuart Pearson wrote in a note. "We add another 4 billion euros in recall cost and fear a harsher commercial impact," added Pearson, who rates VW "neutral."

Effects of the scandal, which erupted on Sept. 18, have so far been barely reflected in VW sales figures - although Volkswagen was the only German carmaker to report a decline in car registrations in Germany in October.

Shares in Europe's biggest carmaker were down 7.8 percent to 102.46 euros by 0855 GMT.

($1 = 0.9155 euros)

(Reporting by Georgina Prodhan; Editing by Ludwig Burger and Victoria Bryan)