New Hampshire Tire Regulations

266:50 Vehicle Tires. – A vehicle shall be considered equipped with solid rubber, iron, steel, or other hard tires when it has 2 or more of that kind.

Minimum legal tread depth requirement is 2/32 inches.

It shall be unlawful to drive a motor vehicle or trailer upon the ways unless such motor vehicle or trailer is equipped with tires in safe operating condition in accordance with requirements established by the director. No tire mounted on a motor vehicle or trailer shall be deemed to be in safe operating condition unless it meets the visual and tread depth requirements set forth in this subdivision. A spare tire shall not be deemed to be a "tire mounted on a motor vehicle or trailer" for the purposes of this subdivision.

266:51 Method of Measuring Tread Depth. – 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 sipped 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.

266:53 Farm Vehicle Exemptions. – Farm vehicles registered under RSA 261, self-propelled combines, self-propelled corn and hay harvesting machines and tractors used exclusively for agricultural purposes are exempt from the provisions of this subdivision.

http://www.gencourt.state.nh.us/rsa/html/nhtoc/nhtoc-xxi-266.htm