g1smd - 7:51 am on Nov 25, 2012 (gmt 0)

Starting with extensionless example.com/sportscars then moving to example.com/sportscars/BMW and example.com/sportscars/Bugatti means that a request for example.com/sportscars will be automatically handled by the server as an external redirect to example.com/sportscars/ and the second request would then be fulfilled by the file at example.com/sportscars/index.ext

If you're using a CMS and internal rewrites, the code changes for that are also very easy.