Ever since the 1970s, heavy trucks and buses have been allowed to use diesel virtually without regulation, because the fuel enjoyed a special status under clean air policies. But that could finally change after a federal appeals court ruled against oil refiners on Friday, upholding new U.S. EPA regulations to clean up trucks and buses. The rules are among the very few environmental measures enacted by former President Clinton that won the endorsement of President Bush — largely because they are supported not only by environmentalists but also by the auto industry. The regulations require manufacturers of heavy trucks and buses to cut 95 percent of harmful pollutants from tailpipe emissions of diesel vehicles by the 2007 model year. They also require oil refiners to reduce the level of sulfur in diesel by 97 percent. The result would be a 2.6 million-ton decrease in smog-causing nitrogen oxide emissions and an 11,000-ton decrease in particulate matter per year; 8,300 premature deaths would also be prevented annually, according to the EPA.