The pecking order among the world's largest tire makers changed very little last year - Bridgestone Corp. claimed top honors for the sixth straight year ahead of Group Michelin, Goodyear and Continental A.G. The differentiating factor? Variances in the exchange rates of the major currencies vs. the U.S. dollar played a measurable role in the absolute numbers. This was reflected, for instance, in the estimated value of the global market, which declined marginally from 2012 to $187 billion, and in the seeming incongruous swings in the dollar-denominated sales for various manufacturers.