Ford 1996 Thunderbird Manual page 254

Rotating the tires
Replacing the Tires
Replace any tires that show wear bands. When
your tire shows a wear band, it has only 1/16
inch (2 mm) of tread left.
A worn-out tire
Because your vehicle's tires may wear unevenly,
you may need to replace them before a wear
band appears across the entire tread. Some spots
wear more heavily than others.
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