Sarah Jane Smith owned several cars.
Sarah Jane aquired the car between the events of DW: School Reunion and SJA: Invasion of the Bane as she had a Volkswagen in the former episode.
The car was used by Sarah Jane to transport her friends and family to various alien sightings. She was also in the car when she was stopped by a Dalek and almost exterminated (DW: The Stolen Earth).
First car
Sarah Jane owned a yellow VW Beetle. She later gave it to Luke when he left for Oxford University. (SJA: The Nightmare Man)
Second car
When Sarah Jane was investigating a school, she had a silver car. She kept K9 in the boot. Mickey Smith used it to crash into and open up the doors to the school when they were locked by the Krillitane. (DW: School Reunion)
Third car
After meeting the Doctor at the school, Sarah Jane obtained another car which she would use throughout her adventures with Luke, Clyde, Maria and Rani. The registration number was originally J590 WAX, (SJA: Invasion of the Bane) after which its registration number became J337 KAE.