1999 GMC Sonoma Owner's Manual - Page 284

yellowblue When It's Time for New Tires CAUTION: Rust or dirt on a wheel, or on the parts to which it is fastened, can make wheel nuts become loose after a time. The wheel could come off and cause an accident. When you change a wheel, remove any rust or dirt from places where the wheel attaches to the vehicle. In an emergency, you can use a cloth or a paper towel to do this; but be sure to use a scraper or wire brush later, if you need to, to get all the rust or dirt off. (See "Changing a Flat Tire" in the Index.) One way to tell when it's time for new tires is 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 new tire if any of the following statements are true: D You can see the indicators at three or more places around the tire. tire's rubber. D You can see cord or fabric showing through the D The tread or sidewall is cracked, cut or snagged deep enough to show cord or fabric. 6-46