Sarah Jane Smith owned several cars.
First car
Sarah Jane owned a yellow VW Beetle with registration JTB 329K. She later gave it to Luke when he left for Oxford University. (SJA: The Nightmare Man)
Luke later used it to return to London when Sarah Jane and Clyde went missing (SJA : Goodbye Sarah Jane Smith)
Second and Third cars
In K9 and Company, Sarah Jane drove a light blue Austin Metro cabriolet, with the registration "OKR 591W". When Sarah Jane was investigating a school, she had a silver VW Passat estate with the registration R74 CGK. 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)
Fourth car
After meeting the Tenth Doctor at the school, Sarah Jane obtained 1991 Nissan Figaro which she would use throughout her adventures with Luke, Clyde, Maria and Rani. Its first UK registration number (all Figaros were sold new to retail consumers in Japan) was J590 WAX, (SJA: Invasion of the Bane) after which its registration number became J337 KAE.