NEW YORK (CNNMoney.com) - The two most efficient auto assembly plants in North America are a Ford plant in Atlanta and a General Motors plant in Ontario - and both of them are going to be closed. Rankings of auto plant efficiency were released Thursday by Harbour Consulting, a widely respected Michigan research firm used by the automakers to judge their plants' productivity. The Ford Atlanta Assembly plant, due to closure in the second half of 2006, was the most efficient North American plant in 2005, according to rankings released Thursday. The good news for GM (Research), Ford (Research) and...