Washington Revised Code Title 46. Motor Vehicles § 46.44.042. Maximum gross weights--Axle and tire factors

Subject to the maximum gross weights specified in RCW 46.44.041, it is unlawful to operate any vehicle upon the public highways with a gross weight, including load, upon any tire concentrated upon the surface of the highway in excess of six hundred pounds per inch width of such tire. An axle manufactured after July 31, 1993, carrying more than ten thousand pounds gross weight must be equipped with four or more tires. An axle carrying more than ten thousand pounds gross weight must have four or more tires, regardless of date of manufacture. Instead of the four or more tires per axle requirements of this section, an axle may be equipped with two tires limited to five hundred pounds per inch width of tire. This section does not apply to vehicles operating under oversize or overweight permits, or both, issued under RCW 46.44.090, while carrying a nonreducible load.

The following equipment may operate at six hundred pounds per inch width of tire: (1) A nonliftable steering axle or axles on the power unit; (2) a tiller axle on firefighting apparatus; (3) a rear booster trailing axle equipped with two tires on a ready-mix concrete transit truck; and (4) a straddle trailer manufactured before January 1, 1996, equipped with single-tire axles or a single axle using a walking beam supported by two in-line single tires and used exclusively for the transport of fruit bins between field, storage, and processing. A straddle trailer manufactured after January 1, 1996, meeting this use criteria may carry five hundred fifteen pounds per inch width of tire on sixteen and one-half inch wide tires.

For the purpose of this section, the width of tire in case of solid rubber or hollow center cushion tires, so long as the use thereof may be permitted by the law, shall be measured between the flanges of the rim. For the purpose of this section, the width of tires in case of pneumatic tires shall be the maximum overall normal inflated width as stipulated by the manufacturer when inflated to the pressure specified and without load thereon.

The department of transportation, by rule with respect to state highways, and a local authority, with respect to a public highway under its jurisdiction, may extend the weight table in RCW 46.44.041 to one hundred fifteen thousand pounds. However, the extension must be in compliance with federal law, and vehicles operating under the extension must be in full compliance with the 1997 axle and tire requirements under this section.