Well my Zonda R can get under 6 minutes. Maybe you need more practice on that particular car.
The Mercedes CLK LM Race Car runs low 5:50's easy
Mclaren F1 GTR Race Car also
Viper GTSR Team Oreca bit slower but it'll do it
SuperGT Cars like Honda HSV Weider, Nissan GTR (whichever one hits 650, I use the stealth)
Also, any LMP car will hit 5's really easy. But I consider them cheater cars at 650
RM tyresPeugeot 905 is the fastest 650pp car on the ring.
5:48 by mike_gt3 (the best driver on the ring)
If the Furai reaches 650pp try it. It jumps to mind immediately.
De-tuning LMP's will easily get you in 6minutes but it's kind of cheating, haha. The GT-R is pretty much OP on this game.
The Furai is one of my favorite cars, so I have a lot of practice, but my best time is 6:15, and granted 15 seconds is a lot of time, but I'm sure a better driver could get it done.Dont think the furai could get sub 6 minutes.