Following the conclusion of the Second Barsoom War, the diffusion of alien technology into consumer goods accelerated exponentially. Martian technomancy was adapted to make grav-resist vehicles not only possible but practical as well.
Here we see the 1927 Calumet Sixty Special, one of the first generation of civilian grav cars to use the new Lileks-Janney Composite Drive. While most examples were built on Earth, it was also manufactured under license by Borgward at their New Dearborn facility on Telemachus.
This particular example was made famous by its use by the Butler Gang on their string of high-profile crimes across southern Italy. They were never brought to justice, despite the best efforts of law enforcement across all the Worlds and Parallels. It was speculated that unsafe performance modifications made to the Lileks-Janney Drive caused a quantum implosion event, but this is mere guesswork. Even now treasure hunters comb the depths of the Adriatic, looking for the lost fortune in black diamonds and Venusian paramercury.