The Star Car is a civilian space vehicle.
Centauri drives a Starcar that can go at least 303 miles per hour (according to the police car radar) and has rocket boosters that allow it to travel through space. It also has some sort of space warp capabilities evidenced by the fact that it is able to leave the solar system and arrive at a planet in a different star system in a relatively short period of time.
The Starcar has gull-wing doors and a 1984 California license plate that says RYLOS. It appears to be in somewhat rundown condition, since it takes Centauri a couple of tries to fire the rocket boosters. Also, it breaks down when Centauri returns Alex to Earth.