Sarah Jane Smith owned several cars from the 1970s to the 2010s.
First car
During her affiliation with UNIT, Sarah drove a gold MG Midget. It had the UK registration plate RMF 654L. She used it while she went to investigate the group Think Tank. (TV: Robot)
Second car
Sarah Jane Smith had a Volkswagen Beetle she called "Ethel," which was blown up. (AUDIO: Comeback)
Sarah Jane owned a yellow VW Beetle with the registration JTB 329K. She later gave it to her son Luke Smith when he left for Oxford University with K9. (TV: The Nightmare Man)
Luke later used it to return to London when Sarah Jane and Clyde Langer went missing. (TV: Goodbye, Sarah Jane Smith)
Whether Luke's Beetle is Ethel (repaired) or another Beetle is unknown.
Third, Fourth, Fifth and Sixth cars
In 1981, Sarah Jane drove a light blue Austin Metro cabriolet, with the registration OKR 591W. (TV: A Girl's Best Friend)
In 1995, Sarah Jane Smith drove an MG car. (HOMEVID: Downtime)
Sarah Jane Smith had a Land Rover in 1996. (PROSE: Interference - Book One)
When Sarah Jane was investigating Deffry Vale High School in 2007, she had a silver VW Passat estate with the registration R74 CGK. She kept K9 in the boot. Mickey Smith used it to crash straight through and open up the doors to the school when they were locked by the Krillitane. (TV: School Reunion)
Sixth car
After meeting the Tenth Doctor at Deffry Vale, Sarah Jane obtained a 1991 Nissan Figaro which she would use throughout her adventures with Luke, Clyde, Maria Jackson, Rani Chandra and Sky Smith. (TV: Invasion of the Bane, TV: Revenge of the Slitheen, et. al) Its first UK registration number was J590 WAX, (TV: Invasion of the Bane) after which its registration number became J337 KAE (TV: The Stolen Earth).