Senate Democrats are planning to include a considerable boost in fuel economy regulations as part of an energy bill being prepared for debate and vote on the Senate floor. If the proposed language becomes law, automakers would be faced with the largest increase in the federal corporate average fuel economy standards, known as CAFE standards, since the rules were created in the 1970s. Under current law, all new passenger cars sold in the United States must get an average of 27.5 miles per gallon. Light trucks, including sport utility vehicles, must meet a 20.7-m.p.g. standard. Under the proposal from Sen. Ernest Hollings (D-SC), all vehicles sold by an automaker would be required to meet a 39-m.p.g. average standard that would be phased in by the 2012 model year. The proposal would boost the standards for light trucks to 27.5 m.p.g. and cars to 36 m.p.g. by the 2006 model year, and then require all vehicles to meet an average fleet standard of 36 m.p.g. by the 2010 model year before reaching 39 m.p.g. two years later. That could be a significant challenge for automakers, according to industry experts. While all three of the major U.S. automakers met the current guidelines for 2001 passenger cars, Ford Motor Co. was the only one to meet the light truck standard, according to data from the National Highway Traffic Safety Administration (NHTSA).