Section 265:60 Basic Rule and Maximum Limits.

A speed limit sign doesn't mean you cannot speed up.   

In New Hampshire, you are generally allowed to temporarily exceed the posted speed limit when passing another vehicle, provided it is done safely and within the bounds of the state's "basic speeding law." According to New Hampshire Revised Statutes (RSA 265:60), the basic speeding law prohibits driving at a speed "greater than is reasonable and prudent under the conditions and having regard to the actual and potential hazards then existing."

This means that while passing, you must ensure your speed remains safe given the road conditions, traffic, and visibility.  New Hampshire uses "presumed" or "prima facie" speed limits rather than absolute ones, which allows some flexibility. If you exceed the posted speed limit while passing, you are not automatically guilty of speeding, as you can argue in court that your speed was safe under the circumstances (e.g., clear visibility, no oncoming traffic, and safe road conditions). However, you must return to an authorized lane of travel as soon as practicable and, if passing involves crossing the center line, ensure the left side is clear of oncoming traffic for a sufficient distance (at least 200 feet from any approaching vehicle) (RSA 265:7).