European carmakers are at last gaining traction in their decades-long campaign to persuade American car buyers to switch from petrol to diesel. Volkswagen boosted sales of diesel models 46.2 percent last month compared with a year earlier, more than double the increase in its overall U.S. sales. Diesel-powered models make up only about 2 percent of cars and light trucks on U.S. roads, compared with more than 50 percent in Europe.