This is just one of many articles, but they all pretty much say the same thing. Nissan used slicks and possibly a tuned version on the GTR. According to Porsche they were not able to get a time within 25 sec

of the claimed 7:29! Calling bullshit on the GTR is not new, but this is another automobile manufacturer saying it's not possible
http://www.autoblog.com/2008/09/30/p...ting-the-ring/