1997 Buick Riviera Owner's Manual - Page 283

When It's Time for New One way to tell when it's time for new tiresis to check the treadwear indicators, which will appear when your tires have only 1/16 inch (1.6 mm) or less of tread remaining. You need a newtire if any of the following statements are true: a You can see the indicators at three or more places around the tire. a You can see cord or fabric showing through the tire's rubber. The tread or sidewall is cracked, cut or snagged deep enough to show cord or fabric. The tire has a bump, bulge or split. 6-46