U.S. highway safety regulators on Nov. 21 opened an investigation into light-truck tires made by Goodyear Tire & Rubber Co. that have been linked to 15 deaths and 129 injuries, according to Reuters. The National Highway Traffic Safety Administration (NHTSA) said it had received 37 reports of tread separations, including 31 crashes, involving the heavy duty, load range ``E'' tires that Goodyear has been manufacturing for the past decade. The investigation comes three months after the recall of 6.5 million Bridgestone/Firestone Inc. tires linked to 119 U.S. highway deaths. Goodyear spokesman Chris Aked said the company stood by its product. ``We're very confident about the performance and durability of the tires,'' he said. The company said its own investigation of tread separation problems led to an internal engineering analysis that found no material or design defect. Nevertheless, the tire maker began adding nylon overlays in 1996 to increase tire strength in response to the growing weight and higher speeds of commercial vans, pickup trucks and large sport-utility vehicles. ``Consumers were putting more stuff in the vehicles, but not going to larger tires that were more durable,'' Aked said. NHTSA said about 21 million Goodyear Load Range E tires of various sizes and names were under investigation. The company said it had manufactured more than 27 million with about half still on the road.