U.S. motorists’ habit of filling up with costlier “premium” gasoline when pump prices drop is delivering extra profits to refiners, such as Royal Dutch Shell and traders like Noble Group. For the second time in the past decade, a sharp fall in pump prices has triggered a spike in demand for the higher-octane fuel that has far outpaced the overall rise in consumption. In the first four months of the year, sales of premium grade gasoline – which makes up around a tenth of the market – surged by 12.6 percent, according to the latest data available from the U.S. Energy Information Administration.