This is a summary. To read the whole story subscribe to BostonGlobe.com
General Motors and the other Detroit automakers reported mixed results for December. GM sold 230,000 new cars and trucks, a 6.3 percent decline from December 2012. But for the year, it reported an overall gain of 7.3 percent in sales, and said that all four of its brands — Chevrolet, Buick, Cadillac, and GMC — improved their retail market shares. GM’s top performers for 2013 were some of its freshest products, like the Cadillac ATS luxury sedan and the Chevy Spark minicar.