Roger Rodas, Walker's friend and financial adviser, was driving the 2005 Porsche Carrera GT at an unsafe speed, and witnesses interviewed by deputies estimated it was going 100 mph or more.
No alcohol or drugs were detected in the system of either man on the day of the fiery one-car crash.
The Nov. 30 deaths were ruled accidents and were due to combined traumatic and thermal injuries, the report said. It said both men were burned over 100 percent of their bodies.
Rodas had fourth-degree burns on his head and neck and severe skull fractures, the report said.
Walker had broken bones throughout his body including his jaw, arm, ribs and pelvis, it said.
Investigators found no mechanical problems with the Porsche or debris or other problems on the roadway in an industrial park in Santa Clarita about 30 miles northwest of downtown Los Angeles.
The car slammed into a tree and a light pole on the driver's side then after spinning, hit a second tree on Walker's side of the car and caught fire.
The report says the red Porsche was traveling "at an unsafe speed, approximately 100+ mph," according to a deputy who took testimony from witnesses at the scene.
"For unknown reasons, the driver lost control of his vehicle," the report says.
The Sheriff's Department had previously cited speed as a factor in the crash, but had released no estimate of how fast the Porsche was going.
Sheriff's investigators are working with Porsche officials and the California Highway Patrol to determine the speed more exactly. Three data recorders survived the crash and fire and may produce information to pinpoint the speed.
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 occurred while Walker was on a break after shooting about half of "Fast & Furious 7," whose release Universal Pictures has now delayed for almost a year to April 2015.
Walker still will appear in the film, though Universal has not said exactly how it will handle his unfinished performance.
Also Friday, two men pleaded not guilty to stealing a roof panel of the wrecked Porsche from the tow truck removing it from the accident scene.
Anthony Edward Janow, 25, and James Brooks Witty, 18, were each charged with felony counts of grand theft and misdemeanor counts of destroying evidence and resisting or obstructing a peace officer. They are due to return to court Feb. 25.