You should monitor your tread depth closely once it reaches 4/32 inch deep. Here's how to assess your tread depth:
4/32" or deeper Good
3/32" Replace Tires Soon
2/32" or less Replace Tires Now
There are several popular ways to check your tire tread depth. One easy way is the penny test. Simply insert a penny into your tire's tread groove with Lincoln's head upside down and facing you. If you can see all of Lincoln's head, your tread depth is less than 2/32 inch and it's time to replace your tires.
Another easy coin test is the quarter test. Insert a quarter into your tread groove. If the tread touches Washington's head, you have at least 4/32 inch of tread remaining.
Don't have any change on you? No problem. Another way to check tread depth is to look at the treadwear indicator bar that's molded into your tires. The bars are located at the bottom of the tread grooves in several locations around the tire. When these bars become visibly flush with the adjacent ribs the tire has no more than 2/32" of tread remaining. This is a visible indication that the tire should be replaced.
Keep your car in good condition to prevent you from filing insurance claims!
jake [at] brodbeckporter [dot] com