1more point - Most people here talk about tread depths in inches, not millimeters. (My depth gauge only shows inches, too.)

 

Regardless of wear bars and tread depth. If you see cord you are walking on hallow ground.

 

FYI: Rears wear about twice as fast as fronts. There are transverse "wear bars" in tread (run your finger down a groove and you will feel these). The tires get very noisy when run to this depth. Or you can use George Washington's head conveniently located on a quarter. (How's that for "Historical"?) When you can see the top of his head you change them. Watch for uneven wear across the tire and check at the lowest point. By the time you are "slipping" you're pressing your luck. I get about 12 k miles on the rears. You may or may not.