To help you to understand - this setting only reacts to pages that are still in the tabs table, but no longer show up in the site - or, to put it another way, if they page is in the recycle bin on the site, it will redirect or emit a 404 error.
This is not the same as an all-purpose 404 error handling application, which the module cannot do - it passes Urls onto asp.net, which then decides if the url is a 404 or not.
The reason /default.aspx?tabid=234234234 responds is that it is a valid url as far as asp.net/iis is concerned, just not as far as DNN is concerned. This type of Url is not actually rewritten, so the Url Master module ignores it (it doesn't try and verify existence or not of tabs when requested directly, it just rewrites the url so that dnn recognises the /default.aspx?tabid=xx pattern).
You can get a redirect on the home/auto/cars/tabid/755 url if you enter it as a custom url for the page you'd like to redirect to. To do this, go to the admin-page urls page, select the page you'd like to redirect to, and enter a url of /home/auto/cars/tabid/755/default - then select the 301 redirect status, and apply changes. This will intercept that Url and forward you to the correct page. IT has to be like this for precedence reasons.