With the onset of the rainy season, now is a good time to check the wear on your tires. Most modern passenger car tires should have a treadwear indicator bar. This typically indicates a depth of 2/32″ (1.6mm) which is the minimum legal depth for tread in the US. If the tread on your tire is level with this indicator you should change your tires immediately.
If the tread on your tire is level with the tread indicator bar, then change your tire immediately
Newer information indicates that you should replace your tires when they reach a tread depth of 4/32″ (3.2mm)
http://www.tirerack.com/winter/tech/techpage.jsp?techid=157
If you Google checking your tire tread depth, you will most likely come across the “penny test“. This test is useful if you want to check using the standard of 2/32″ (1.6mm) providing you have a US penny handy.
If you are concerned about traction in the wet and prefer to change your tires at the 4/32” (3.2mm) mark then you can use a Trinidad & Tobago 25 cents coin.
The bottom of the writing is 3mm from the bottom edge of the coin. Hold the coin in the tread of the tire. If you can read the word “CENTS” then your tire is worn beyond the 4/32″ (3.2mm) tread depth and you should consider changing your tire.
