Paul Walker -- Speed, Not Mechanical Failure, Caused Fatal Crash

Paul Walker died as a result of speeding and not mechanical problems.

A source connected with the CHP -- which did the speed analysis -- tells TMZ ... Roger Rodas, the driver, was going between 80 and 93 MPH when he lost control going around a curve and crashed.

The Associated Press quotes a source from the L.A. County Sheriff's Dept. -- the agency that conducted the mechanics analysis -- who says the crash was NOT the result of mechanical failure. Speed was the trigger for the fatal accident.

Investigators also noted the tires were 9 years old. Shortly after Paul died ... law enforcement sources told us they were the original tires on the car and they had hardened ... which means it lost traction on the road.

TMZ broke a story in December ... Porsche had issued a memo to its dealers 9 years ago warning them the 2004 Porsche Carrera GT could be dangerous to an untrained driver. The memo says the road surfaces need to be smooth, like a race track. The car is so sensitive, the memo says, "This vehicle cannot drive over a Foster beer can that is lying on its side."

Investigators say there was no evidence a second car was involved in the collision.

They also said an after-market exhaust system had been installed, which would have increased the engine's horsepower. According to investigators, the coroner's office determined neither Walker nor Rodas had alcohol or drugs in his system at the time of the crash.

TMZ broke the story ... Walker and Roger Rodas died Nov. 30 from a combination of the impact and fire.

During the investigation, German Porsche mechanics flew to L.A. to examine the car .... analyzing the Carrera GT's brakes, engine, body and a tire that wasn't destroyed in the fire.