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Tread is in need of maintenance when: There are wear bars unevenly on the tire; if there are wear bars exclusively on the outsides OR insides of the tires (can be something as simple as improperly or unevenly balanced tires, or something more complex as the front end needing an alignment).
Regardless whether you have a car with multiple belts or a vehicle with a serpentine belt (a singular belt that wraps and winds into multiple areas of the engine), the belt(s) need to be checked for cracking, dry rot, edge wear or excessive wear down that causes a shiny outer area, which allows for slippage.