Houston Chronicle: The federal government imposed the first national limits on greenhouse gas emissions Thursday with the tightening of fuel efficiency standards for cars and light-duty trucks. The new rules, finalized by the Environmental Protection Agency and Transportation Department, require a fleet average of 35.5 miles a gallon by 2016 — up from an average 27.5 miles per gallon today. Officials said the tougher tailpipe standards would save consumers money at the gas pump, reduce the …