Investigators found no mechanical problems with the 2005 Porsche Carrera GT or debris or other problems on the road in Santa Clarita, about 30 miles north west of central Los Angeles.
The street forms about a one-mile loop amid industrial office parks and is rimmed by hills and isolated from traffic, especially at weekends. The downed lamppost the car struck had a speed limit sign of 45mph.
Roger Rodas, Walker's friend and financial adviser, was driving the Porsche at an unsafe speed and witnesses interviewed by deputies estimated it was travelling at 100mph or more.
No alcohol or drugs were detected in the system of either man on the day of the fiery crash on November 30.
The deaths were ruled accidents due to combined traumatic and thermal injuries, the report said. It said both men suffered 100% burns.
Mr Rodas, 38, had fourth-degree burns on his head and neck and severe skull fractures and Walker, 40, had multiple broken bones.
The car slammed into a tree and the lamppost on the driver's side, then, after spinning, hit a second tree on Walker's side of the car and burst into flames.
The report says the red Porsche was travelling "at an unsafe speed, approximately 100+ mph", according to a deputy who took statements from witnesses at the scene.
"For unknown reasons, the driver lost control of his vehicle," the report says.
Investigators are working with Porsche and the California Highway Patrol to determine the speed more exactly. Three data recorders survived the crash and fire and may produce information.
Rodas, 38, and Walker, 40, co-owned an auto racing team. Rodas also was a professional driver who competed in 10 Pirelli World Challenge GTS races last year.
The accident happened while Walker was on a break after shooting about half of Fast & Furious 7. Universal Pictures has now delayed the release for nearly a year until April 2015.
Walker still will appear in the film, though Universal has not said exactly how it will handle his unfinished performance.
* Two men have denied stealing a roof panel of the wrecked Porsche from the tow truck removing it from the accident scene.
Anthony Janow, 25, and James Witty, 18, were each charged with felony counts of grand theft and misdemeanour counts of destroying evidence and resisting or obstructing a peace officer. They are due to return to court on February 25.