Volkswagen Group sales rose 0.6% in 2008 to a record 6.23m vehicles but chief executive Martin Winterkorn has warned that volume could fall as much as 10% this year, in a global market that could be down by around 20%.

Sales in China topped 1m for the first time. “We have kept our promise: the Volkswagen group delivered more vehicles than ever before in the difficult year of 2008,” Winterkorn said as Detroit show opened to media.

Not all brands fared equally. VW sales rose 0.1% to 3.67m, Skoda was up 7.1% and Lamborghini sales rose 1% to 2,430 units. But Seat was down 14.6% to 368,1000 units and Bentley was off 24.1% to 7,605.

Winterkorn said construction of the group’s  new US plant in Chattanooga, Tennessee was on schedule and that the first cars would be produced there from spring 2011.

This is expected to be a new mid-sized car designed for North America but Winterkorn gave no details.

However, he warned that 2009 would be an extremely difficult year for the auto industry and said his company would focus on attractive, economic and clean vehicles suited to the times.

He said the relatively young and fuel efficient product range should help VW emerge from the credit crisis stronger than before and predicted that there would be a focus on hybrid and electric drive trains and second generation biofuels.

Volkswagen revealed a roadster concept called Concept BlueSport in Detroit.