Measuring Tire Tread Depth?
Tread depths need to be checked and recorded because DOT has a legal limit -4/32” for steer tires and 2/32” of rubber for drive and trailer tires. That means that if any spot in the major grooves of a street tire are found to be 4/32” or less, that tire must be removed from service.
However, it can either be retreaded or moved to the trailer and run down to 2/32" minimum before entering the retreading process. Drive tires would be required to be removed from service or retreaded when any major groove reaches 2/32” of rubber to be safe. Most fleets have found over the
years that running the tire down to 2/32”.
The last tip about measuring tread depth: never measure in the decoupling groove which is found in many steer tire designs to reduce irregular wear. This outside decoupling groove is sometimes known as a PDG groove (Pressure Distribution Groove) and they may be very deep or may be very shallow–but it is not the same depth as the major tire grooves.