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