A Demo Car is any car that was exclusive for some demo version, that is, a car that isn't present in no other Re-Voltversion except by the demo where it belongs to. These cars were made for sponsorship, with the advertisement aimed to the company or industry that promotes the game or distributes the demo. For instance, GameSpot distributed the demo v0.01gamespot, and the demo car this demo has is called Gamespot.
Demo cars are extra cars in the boxes stack. They usually replace the box of Humma or just appear after the Panga box (where special and custom cars are located in the car selection screen). Their box art is the default one that is used for custom and special cars, which is a car covered with a white sheet with red question marks at the background. They are never selected by the CPU and are usually electric-poweredrookies, since the only rating available in the demos is the rookie.
These cars were part of the strategy of the game marketing department. They were made to attract the public or companies and make them interested in promoting the game. Not all of the ideas were successful though.
Even though cars like Sprinter XL and RC Bandit are selectable in demos, they're not considered demo cars because they are not exclusive to some demo.
↑Excerpts of Joe Groombridge reply to the email of the revolter Stefano: "I think our marketing department were looking at different angles to get our game seen and talked about. Most of these ideas never happened but the artwork is still floating about. [...] I was asked to do some alternative skins for existing cars that were tie-ins with gaming magazines to try to get them excited about the game."