Exxon Mobil Corp. (XOM)???s push to export U.S. oil overseas is facing a new hurdle: falling gasoline prices. A surge of new oil from Texas to the Great Plains has flowed to refineries, driving down prices at the pump 10 percent since March, while global oil prices have hovered at about $107 a barrel. That disparity shows that the influence of the world crude market on U.S. gasoline prices is waning. U.S. supplies are having a greater impact because they???re making up a bigger part of the gasoline market, supplying about 53 percent today, compared with 34 percent less than three years ago.