DETROIT (Reuters) - General Motors Co and Ford Motor Co , the two largest U.S. automakers, have agreed to jointly develop a new line of nine- and ten-speed automatic transmissions to boost the fuel economy and performance of their lineup, the companies said on Monday. The pact marks the third time in the last decade that the two automakers have collaborated on transmissions. The joint effort allows GM and Ford to bring the transmissions to market more quickly and at a lower cost than if they worked alone. ...