When to replace street tires used for the track?
 

Here's a pic for reference of my current Direzza II tires.

http://i.imgur.com/HiU8qeJ.jpg

They're at the wear bars right now. I won't be driving them in the rain. Can I still drive these at the track without worrying about the safety of the tires? Obviously maximum grip isn't a concern if I'm driving them this low.

You may have some life left in them unlike these.....

http://i39.tinypic.com/dmabzc.jpg

For dry track?

As long as cord isn't showing & reading on one of these is good as well:

http://i.ebayimg.com/t/Tire-Duromete...k(g~~60_12.JPG

Generally what we do - run them until you can see cords, or they stop providing grip. With a street tire, you will see cord before you lose grip.

As soon as you see cord, they are done. Don't go further... steel belts do not grip :)

Keep in mind, always have enough tire for an event. Running out of tire can be expensive if you have to scrap sessions halfway through the day.

Run em till they cord. If you know they're going to cord at your next event, replace them.

Keep in mind grip will drop throughout the tire's life as you heat cycle them with hard driving. Also, it's not legal to drive with balding tires (at least in California), so that's a consideration as well.