Truck and trailer rules in Montana: commercial vehicle line, trailer brakes, size and weight limits

This page reports what the official source says. It is not legal advice: ask the office named here. The sources are sections of the Montana Code Annotated 2025, Title 61, Motor Vehicles, published by the Montana Legislature: chapter 1 on definitions, chapter 9 on vehicle equipment and chapter 10 on size, weight and load. Federal background and the other places are on the parent page, trucks and trailers for tree work.

At a glance

Item	What the sources say
Commercial motor vehicle	Combination or vehicle of 26,001 pounds or more, including a towed unit over 10,000 pounds; 16 or more passengers; a school bus; any size carrying hazardous materials
Licence classes	Not set in the sections read
Trailer brakes	On all wheels above 3,000 pounds gross; automatic on breakaway and held 15 minutes
Width, height	102 inches wide; 14 feet high
Length	55 feet single truck; 75 feet for most combinations; 53 foot semitrailer
Weight	20,000 pounds single axle; 34,000 pounds on two consecutive axles in the spacing range the section words; formula for gross weight

The commercial vehicle line

Section 61-1-101 defines a commercial motor vehicle as a motor vehicle or combination used in commerce to carry passengers or property that has a gross combination weight rating or weight of 26,001 pounds or more including a towed unit rated over 10,000 pounds, has a gross vehicle weight rating or weight of 26,001 pounds or more, is designed to carry at least 16 passengers including the driver, is a school bus, or is of any size and carries hazardous materials. The definition excludes authorized emergency vehicles on a call and certain other listed vehicles. See section 61-1-101 on definitions.

Brakes

Section 61-9-301 requires every motor vehicle, trailer, semitrailer, pole trailer and combination on a highway to have brakes that comply with part 3 of chapter 9. Section 61-9-304 requires brakes acting on all wheels, with exceptions that include a trailer, semitrailer or pole trailer of a gross weight not over 3,000 pounds, provided the total weight on its wheels does not exceed 40 percent of the gross weight of the towing vehicle and the combination can meet the stopping standard. Section 61-9-305 says every trailer, semitrailer and pole trailer with air or vacuum brakes, and every one with a gross weight over 3,000 pounds made or assembled after January 1, 1966, must have brakes on all wheels that apply automatically and promptly, and remain applied for at least 15 minutes, on breakaway from the towing vehicle. Section 61-9-312 sets the stopping distance from 20 miles per hour on dry level road at 40 feet for trucks and tractor trucks and for all combinations of vehicles, and 25 feet for passenger vehicles. See section 61-9-304 on brakes on all wheels and section 61-9-305 on breakaway.

Width, height and length

Section 61-10-101 says sections 61-10-102 to 61-10-104 and 61-10-106 to 61-10-110 set the maximum dimensions and weights to the exclusion of other standards. Section 61-10-102 limits a vehicle, unloaded or with load, to a total outside width of 102 inches, with exceptions including implements of husbandry moved in daylight for farming. Section 61-10-103 limits height, unladen or with load, to 14 feet, with a 15 foot 6 inch allowance for hay and straw hauling. Section 61-10-104 limits a single truck, bus or self-propelled vehicle to 55 feet overall including bumpers. A semitrailer in a truck tractor-semitrailer combination may not exceed 53 feet, and in a three-unit combination each trailing unit may not exceed 28 1/2 feet or 61 feet combined. All other combinations may not exceed 75 feet, and if a combination has more than two units the rear units must have breakaway brakes. The section also limits a motor vehicle to towing one motor vehicle, except for triple saddle-mount combinations. See section 61-10-102 on width and section 61-10-104 on length.

Weight

Section 61-10-107, which the legislature's site labels as the temporary version, says an axle may not carry more than 20,000 pounds, and no two consecutive axles "more than 40 inches or less than 96 inches apart" may carry more than 34,000 pounds, in the section's own wording. An axle load is the load carried by all wheels whose centres lie between two vertical planes 40 inches apart, and axles 40 inches or less apart count as one. Maximum gross weight on a vehicle, group of axles or combination follows the bridge formula W = 500((LN/(N - 1)) + 12N + 36), with two consecutive sets of tandem axles allowed 34,000 pounds each if the distance between first and last axles is 36 feet or more. The section also says gross weight may not exceed the limits the Department of Transportation adopts by rule, based on 23 CFR part 658, appendix C. Except for the steering axle, all axles over 11,000 pounds need at least four tires or wide-base tires, and an axle with wide-base tires is limited to 500 pounds per inch of tire width. See section 61-10-107 on maximum gross weight.