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In what may be a Pyrrhic victory, Anglo-Dutch oil supermajor Royal Dutch Shell plc (NYSE: RDS-A) is the market leader among branded gasoline sellers in the U.S. for the first half of 2013. BP plc (NYSE: BP) places a distant second.

That’s the good news. The bad news is that branded gasoline sales have lost overall market share so far this year, down from 66.5% in the first half of 2012 to 64.7% this year.

Shell’s total U.S. market share in the first half of the year was 14.8%, more than double the share of second place BP, which came in at 6.8%. BP’s share dropped from 7.1% in the first half of 2012.

According to a report from the Oil Price Information Service (OPIS), unbranded gasoline picked up market share because it charges less to fill up. On average, branded gasoline is priced higher than competitors’ by 1.51 cents a gallon, while unbranded gasoline is priced at a discount of 2.39 cents a gallon. Interestingly, the premium grew at branded stations year-over-year, while the discount grew at unbranded stations.

Shell was the best-selling branded gasoline in nine states, while Chevron Corp. (NYSE: CVX) led sales in six states and Exxon Mobil Corp. (NYSE: XOM) and International Speedway Corp. (NASDAQ: ISCA) tied for third place, leading in four states each.

And the cheapest gasoline? It’s available at Costco Wholesale Corp. (NASDAQ: COST), where you can fill up for 13.9 cents a gallon less than at any of its competitors, which include Arco (now owned by Tesoro Corp. (NYSE: TSO)), Wal-Mart Stores Inc.’s (NYSE: WMT) Sam’s Club stores, and privately held BJ’s Wholesale Club.