Black Circle tyre testWhile officially 2/32nd's is when a tire is worn out. I would suggest you watch this video on youtube that does a wet breaking test using a new tire, a tire with 4/32nd's, and then 2/32nd's. Safety First!

I've looked through my Tread Design Guide and couldn't find it, however if you want...I can find out who makes it by the first two letters or numbers of the DOT (Department of Transportation) number and I'd be more than happy to assist.