A. This depends on type of racing and vehicle the tire is used for. In certain sportsman classes, racers have completed over 150 passes on one set. High horsepower and heavier vehicles tend to more quickly wear out the slicks. Inconsistent 60' time is an indicator that the slick is "worn out", even before the tread depth indicators are gone. The tread depth indicators are 'dimple' cavities on the tread surface that help monitor wear. Tread depths vary by size, please refer to the Tech Specs on the Size & Specs tab of this product page for details. The tire is considered to be worn out as soon as the indicator depth is zero. Constantly monitor tread depth. Immediately discontinue use if any of the cord material becomes visible.