Tesla sales in the United States decreased about 9 percent in the first three months of the year, even as the total market of electric cars, according to the data collected by a research company. Cox said that sales of all electric cars in the United States increased by 11 percent during the first quarter to about 300,000 light cars and trucks, much faster than the total car market, which was fixed. The company said: “Despite many obstacles – and what you may read elsewhere – sales of electric cars continue to grow at a health pace in the American market,” the company said.