Sorry, your title was misleading; I think you corrected it and said it's ONLY fictionCar/s.com you have, and you don't have carFiction.com?
Car fiction would have had little value undev'd, unless someone thought it was ideal to post fictional car stories on... not a huge market though.
fictionCar/s.com would have to be an entirely different usage; kind of like 'fictional' cars - site about autos that were used in movies/tv shows might be nice; also include prototype models, early car designs that never made it...
undeveloped I'd say no value; and it's not an exciting name, but could be a very clear and descriptive name if used for the abovementioned development. If you're a car buff, you could make a very interesting site with that content, and of course the price would then become substantial if you have a lot of traffic and sold the site complete. I'd go for the developing here...