Study by Berkeley professor largely mirrors Firestone's own probe.
Critics say report adds nothing new and fails to address risks of models still in use.
The Firestone tires that tore apart at high speeds, causing dozens of fatal accidents, lost their treads for a variety of engineering and usage reasons, according to the independent expert who released his report on the tires' problems Friday. The tire failures have been linked to at least 148 highway deaths in the United States and dozens more overseas. Most of the recalled tires were sold on Ford Explorer sport-utility vehicles. Sanjay Govindjee's report largely mirrored Firestone's own probe released Dec. 19. Both said de-treading was caused by small cracks that spread between the tires and the steel belts around them.