RSA 259:115-a: Tridem.

"Tridem" shall mean 3 consecutive axles, the centers of which, when measured between the first and third axles, may be included between parallel vertical planes spaced more than 8 feet apart, and which tridem unit measures less than 12 feet overall between the centers of extreme axles, extending across the full width of the vehicles; except that for vehicles being driven on the interstate and defense highway system with a gross weight in excess of 73,280 pounds, a tridem shall measure not less than 40 inches nor more than 96 inches between the parallel vertical planes.