Montana Title 61. Motor Vehicles § 61-10-107. Maximum gross weight

(1)(a) An axle may not carry a load in excess of 20,000 pounds, and no two consecutive axles more than 40 inches or less than 96 inches apart may carry a load in excess of 34,000 pounds. An axle load is the total load transmitted to the road by all wheels whose centers are included between two parallel transverse vertical planes 40 inches apart, extending across the full width of the vehicle. For purposes of this section, axles 40 inches or less apart are considered to be a single axle. Except as provided in subsection (1)(b), the maximum gross weight allowed on a vehicle, group of axles, or combination of vehicles must be determined by the formula:

W = 500((LN/(N--1)) + 12N + 36)

in which W equals gross weight, L equals wheelbase in feet, and N equals number of axles, except that two consecutive sets of tandem axles may carry a gross load of 34,000 pounds each if the overall distance between the first and last axles of the consecutive sets of tandem axles is 36 feet or more. The maximum gross weight allowed on a vehicle may not exceed the weight limits adopted by the department. The department shall adopt rules for weight limits based upon the most recent version of 23 CFR, part 658, appendix c, for vehicles operating in Montana.

(b) A vehicle traveling on U.S. highway 93 from the border between Canada and the United States to 10 miles south of the border or on Montana highway 16 from the border between Canada and the United States to 20 miles south of the border is subject to the specific maximum allowable gross weight limit provided in rules adopted by the department but is not subject to maximum gross weight limits determined by the formula in subsection (1)(a).

(2)(a) Notwithstanding a vehicle's conformance with the requirements of subsection (1), except for the steering axle, all axles weighing over 11,000 pounds must have at least four tires or have wide-base tires. The maximum load on an axle, other than a steering axle, equipped with wide-base tires is limited to 500 pounds for each inch of tire width.

(b) The provisions of subsection (2)(a) do not apply to passenger buses.

(c) For the purposes of this section, wide-base tires are tires that are 14 or more inches in nominal width. The maximum tire weight limit is computed for wide-base tires based on the number of inches shown on the tire marking, or if the tire marking is shown by metric size, the tire weight limit is computed by conversion of the metric size.