Thus, tire manufacturers use the Tread Wear Indicator (TWI ... These numbers merely denote the tread groove depth in millimeters, with each number corresponding to a specific depth.

If the groove or space does not contain a tread wear indicator, then it cannot be used to determine if the minimum tread depth requirements outlined by 393.75 are being met. Discussion over.

The association says the tread is measured from the bottom of the groove, not the top of the wear indicator bar. That is the purpose of the wear bars, to tell you when the tire has reached minimum ...