Toyota prius Owner's Manual page 441

 When to replace your vehicle's tires
Tires should be replaced if:
 You have tire damage such as cuts, splits, cracks deep enough to
expose the fabric, and bulges indicating internal damage.
 A tire goes flat repeatedly or cannot be properly repaired due to the size
or location of a cut or other damage.
If you are not sure, consult with any authorized Toyota dealer or repairer, or
another duly qualified and equipped professional.
 Tire life
Any tire over 6 years old must be checked by a qualified technician even if it
has seldom or never been used or damage is not obvious.
Low profile tires (vehicles with 17-inch tires)

Generally, low profile tires will wear more rapidly and tire grip performance
will be reduced on snowy and/or icy roads when compared to standard tires.
Be sure to use snow tires or tire chains on snowy and/or icy roads and drive
carefully at a speed appropriate for road and weather conditions.
 If the tread on snow tires wears down below 4 mm (0.16 in.)
The effectiveness of the tires as snow tires is lost.
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4-3. Do-it-yourself maintenance
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