The third vehicle had not been found and the driver had not been identified. Police, who made several public appeals after the tragedy, are still seeking information to help complete the investigation.
Police told the inquiry that the southbound green car tried to overtake into the path of the oncoming Hallgarth.
Hallgarth's Suzuki went out of control and collided with the utility as the green car returned to the side of the road.
The 20-year-old driver of the utility also left before emergency services arrived, because she was unlicensed, but she was not considered to have caused the crash, Scott said.
"[Hallgarth] and the unknown driver of the green Honda contributed to the crash," the coroner said.
"The unknown driver did so by attempting to pass in a dangerous place when he was too close to Ross' car, thereby creating the situation where Ross tried to avoid him and lost control of his car."
"Ross contributed to the crash because his car had four worn tyres."
The tread was illegally thin on three of the tyres.
"I am satisfied this either caused or contributed to him losing control of the Suzuki and fishtailing along the road before colliding with the utility vehicle," the coroner commented, finding death resulted from Hallgarth's multiple injuries.