I really don't have a solid strategy on singulars or plurals as of yet but I do agree with Jim, whereas I think of plurals in a LOT of cases as better.
Here's a quick thought. If I am looking to purchase something online, I think I always type in the plural of that name because I want many choices rather than typing in the singular and most likely getting one choice. It is probably not true since I am sure there a lot of Companies that have the Generic singular name, and do have multiple choices, but I just believe I am getting more options by searching that generic by adding an "s". IMO.
So, in your above example, I would definitely choose "cars".
For some reason, I am thinking about this question a lot. After I responded and jumped out of the thread I was thinking of what few product domains i own, and I have to say I really do like the singular in other cases more than the plural. I have weightbench. Now that in the singular sounds a helluva lot better than the plural would sound.
I wish I could figure out why some generics sound better in plural, while other sound better in singular but I can't. I guess if you are asking this question because you have the option of buying both, then buy them both, and then throw them both in an appraisal thread asking which sounds better. But if it were the case that both WERE available, then you had better buy them both...for traffic, and for a defensive reg so you would eliminate a potential competitor.
Hope that helps a little, lol
Vito.