WASHINGTON (May 31)—Bridgestone/Firestone Inc. formally has asked the National Highway Traffic Safety Administration to open a safety investigation into the Ford Explorer. Testing performed by Dennis Guenther, a professor of mechanical engineering at Ohio State University, "shows that certain of the Explorer models will experience an ´oversteer´ condition in most circumstances following a tread separation on a left rear tire," said Bridgestone/Firestone CEO John T. Lampe in a letter to NHTSA Acting Administrator L. Robert Shelton. Those Explorers, Lampe said, "have an inadequate margin of control to permit control by average drivers in the foreseeable events of tread separation during normal highway driving." Bridgestone/Firestone has asserted design defects in the Explorer helped cause the accidents that killed 174 people. Ford Motor Co., however, insists the Firestone tires on those vehicles were solely to blame.