The United States government is expected to propose an overhaul of fuel economy standards that could pressure carmakers to improve the fuel efficiency of sport utility vehicles, Reuters has reported, citing the Wall Street Journal on Dec. 22. According to the report, the newspaper said that the National Highway Traffic Safety Administration (NHTSA) is targeting such loopholes as DaimlerChrysler's classification of its PT Cruiser as a light truck, making it subject to easier fuel economy rules than a car. Reuters said that, according to the paper, the agency is expected to publish as early as this week a document that will suggest tightening the definition of light trucks -- half of all vehicles sold each year in the United States now fit into this category. Reuters said the Journal reported that the agency also wants to extend the rules to cover the heaviest trucks and SUVs, such as General Motors' Hummer H2, which aren't subject to any fuel economy standards.