Faced with rising gasoline prices, some Americans are trading in their SUVs and gas-guzzling cars for gas-stingy vehicles. The trend, if it holds, would be good news for the environment. The U.S. could reduce its crude-oil imports by 170,000 barrels a day — 62.05 million barrels a year — if consumers switched from SUVs to higher-mileage vehicles, according to a study by the James A. Baker 3d Institute for Public Policy at Rice University. Last Wednesday, meanwhile, the Bush administration canceled a 2004 deadline for automakers to develop prototypes of cars that could get 80 miles per gallon.