Personally I never had a problem running a 6.5 on mine.
To be 100% honest the first time on my astro track the slipper was so tigth i was able to lose the pressure hexes (OT19), simply solved by losening my slipper a bit and tightening a little more the whell nuts
.
Note that I'm running the belt rollers (LA10) over the small pulleys (LA11).
Bye
Ema