Tire tread depth shall be measured by a tread depth gauge which shall be of a type calibrated in thirty-seconds of an inch. Readings shall be taken in a major tread groove of the tire nearest the center at 2 points of the circumference not closer than 15 inches. Readings for a tire which has the tread design running across the tire or for a siped tire, where such tread design is permitted, shall be taken at or near the center of the tire at 2 points of the circumference not closer than 15 inches.