I used the Lincoln Penny method of checking the tread on a set of car tires today, to determine if it is time to replace them:

• Take a penny and place it between the treads, Lincoln’s head first
• If you can see all of Lincoln’s head, your tires have approximately 1/16 of an inch of tread left
• If you only see part of Lincoln’s head, you are okay for now

Found this method online here.