Police said the deceased were identified as Manikandan, his wife Nithya, her mother Saroja, son Dheeran, and daughter Niharika, all hailing from Attur in Salem district.
On being alerted by the passerby, Namanasamudram police rushed to the site and removed the bodies from the car and sent them to Pudukottai Government Medical College Hospital for post-mortem.
Initial probe revealed that the family members committed suicide by consuming poison, owing to huge losses in a business run by Manikandan and mounting debt.
A suicide note was recovered from the car.
Police said the cause of death would be ascertained after an autopsy and further investigations.
