don't forget the driver qualities from that equation! the R34 was driven by one of the ring veterans, there's a video on youtube iirc.
Also some cars are actually engineered around the track when originally developed (again the R34 was one of them) so that is going to make a massive difference in lap times.
I thik you will find that the ring is pretty much like a real road, in that it's pretty bumpy, so gives a good impression of a cars capability. My Cossie would be a nightmare on the ring as te suspension is far too hard, but on a normal track it's perfect.