The Obama administration announced new rules today that would require tighter emissions guidelines for medium and heavy-duty trucks in an effort to reduce greenhouse gases.The rules, proposed by the Environmental Protection Agency and the Department of Transportation's National Highway Traffic Safety Administration (NHTSA), were expected to cut carbon dioxide emissions from trucks and vans by one-quarter by the year 2027.The proposed standards affect semi-trucks, large pickup trucks and vans, buses and work trucks and cover model years 2021-2027, officials said.