Take a penny and put Abraham Lincoln's head into one of the grooves of the tread. If part of his head is covered by the tread, you're ok. If you can see all of his head, it's time to replace the tire. When the tread is worn down to 1/16 of an inch, tires must be replaced. Also remember to check your tire pressures approximately once a week. Low tire pressures can affect fuel economy, handling and comfort. To accurately measure your tire pressure, check tires when they are cold. Wait at least three hours after driving."