All treads have wear bars that run perpendicularly to the tread. when the tread gets worn down so that it's even with the wear bars, you're done.
2006 525 xi, Orient Blue Metallic, Beige leather, Dark Poplar trim, Premium Package, Cold Weather Package
2012 Z4 35i, ED 2/24/12, Melbourne Red Metallic, Black leather, 6 MT, ///
M Sport, Premium Package, Premium Sound Package, Cold Weather Package, Nav, Comfort Access, Alpine HID Angel Eyes, BMWPedals pedal covers, stubby antenna, Extra Enjoyment Package