The girl sustained life-threatening injuries after falling from the vehicle as it travelled along the A180 in Grimsby on Sunday (Aug 22).
Police said the girl was travelling in a black Kia Sedona car which was heading westbound between the Westgate and Pyewipe roundabouts at around 4.45pm when she fell out of the vehicle.
A statement from Humberside Police said: "She has sustained serious and potentially life threatening injuries in the incident and remains in hospital where she is receiving treatment.
"We are appealing for anyone who witnessed the incident or with dash cam footage to get in touch with the officers who are conducting the investigation."
Anyone with information is asked to contact the force on 101, quoting log number 429 of August 22, 2021.