how sloppy are speed tolerences in QIC drives

[compoobah at valleyimplants.com]

I'm trying the vinyl tubing approach in replacing the drive roller, since it sounds like it would be difficult to slide in a different QIC drive when I need to use tape (no one suggested tips on that). How "sloppy" can the drive roller dimensions be? is there a feedback loop for speed regulation that interfaces directly to the tape, or does it rely soley on the stepper motor and parts dimensions? I think I'm about 1/32" too large, but it could be bigger, since it's hard to measure goo, not more than 1/8th, though.