That compared to a death every 94 million miles for all other vehicles on US roads.
A Tesla spokesman said drivers were told to keep their hands on the steering wheel at all times and had ultimate control.
He said: "Autopilot is getting better all the time but it is not perfect and still requires the driver to remain alert.
"In this case neither autopilot nor the driver noticed the white side of the tractor-trailer against a brightly lit sky so the brake was not applied."
According to an accident report the crash happened on a dry road in daylight near Williston, Florida.
Mr Brown went underneath the truck's trailer when it turned left in front of him.
The Tesla's windshield hit the bottom of the trailer as the car passed underneath. It then left the road, struck a fence, crossed a field, hit another fence, and finally collided with a pole.
Frank Baressi, 62, the truck driver involved in the crash, claimed Mr Brown was "playing Harry Potter on the TV screen" at the time and "went so fast through my trailer I didn't see him".
Mr Baressi admitted not having seen Mr Brown watching the Harry Potter movie but said he could hear it.
He said: "It was still playing when he died and snapped a telephone pole a quarter-mile down the road."
Tesla said videos could not be watched on the car's screen. However, it was possible the driver could have been watching another device. There was no mention of a movie in the police report.
Mr Brown had nicknamed his car "Tessy" and posted videos of journeys on the internet, including an incident in April when the autopilot system saved him from a near crash.
At the time he said: "Hands down the best car I have ever owned and use it to its full extent."
Karl Brauer, a senior analyst with Kelley Blue Book, a major US vehicle research company, said the accident would raise public questions about safety.
He said: "They have been touting their safety and they have been touting their advanced technology. This situation flies in the face of both."
But engineering expert Professor William Harwin, of the University of Reading, said Mr Brown's death would not halt the advance of self-driving cars.
He said: "Sadly, Joshua Brown joins others such as William Huskisson who was the first fatality in a train accident in 1830.
"Ultimately, Huskisson's high-profile death did not prevent the widespread adoption of a revolutionary new technology and, I hope, neither will Mr Brown's."
