Montana Tire Regulations

Chains are permissible from October 1 until May 31, but not mandatory unless the department makes that determination and notices are posted. They are then required on one set of drive axles. It is also permissible to use tire chains of reasonable proportions or pneumatic tires (means a tire in which compressed air or nitrogen is designed to support the load.), studded tires, that may not protrude more than one-sixteenth of an inch beyond the tire tread or that are clearly marked by the manufacturer on the sidewall all season mud and snow, upon a vehicle when required for safety because of snow, ice, or other conditions tending to cause a vehicle to skid.

1. A solid rubber tire on a vehicle must have rubber on its entire traction surface at least 1 inch thick above the edge of the flange of the entire periphery.
2. A person may not operate or move on a highway a motor vehicle, trailer, or semitrailer having a metal tire in contact with the roadway.

Because of the many different road and weather conditions throughout our state, MDT along with the local tire industry wants motorists to know there is a variety of snow and studded tire alternatives available so they can choose appropriately. Since different types of winter tires provide varied performance under different road conditions, tire dealers can assist motorists to select the best option for them. A tire on a vehicle moved on a highway may not have on its periphery a block, stud, flange, cleat, or spike, or other protuberance of a material other than rubber that projects beyond the tread of the traction surface of the tire, except that it is permissible to use farm machinery with tires having protuberances that will not injure the highway. It is also permissible to use tire chains of reasonable proportions or pneumatic tires (means a tire in which compressed air or nitrogen is designed to support the load.)

Metric Tires

Vehicles which operate on radial tires, display a metric “Nominal Cross Section” on the side of the tire that can be converted to inches using the following formula: If the nominal width on the tire is in inches, use that figure.

To determine inch width on metric radial “Wide-Based” tires, you must divide the number (tire size) by 25.4 EXAMPLE: Tire size 385-65R 22.5 (Divide385 by 25.4) (385 divided by 25.4 = 15.16) (Tire width is 15.16 inches)

REMEMBER: The 500 lbs per inch width of tire is ONLY used for “Wide Based Tires”. Providing an axle has 4 tires, it is allowed to carry 20,000 lbs.

Allowable load per inch width of tire

With the exception of reduced weight for spring breakup, the allowable weight per inch width of tire (500 lbs.) is to be computed for “Wide Based” tires only. A wide based tire is a tire 14 or more inches in width as shown on the sidewall of the tire, or by using the metric conversion formula.

Single axles with singled out tires cannot exceed 11,000 lbs. unless equipped with wide based tires. Wide based tires cannot exceed 500 lbs. per inch width of tire.

Remember: Steering axles are exempt from this. Manufacturers rated capacity or bridge formula should be used to determine allowable weight on a steering axle.

A single axle with 4 tires can go up to 20,000 lbs. of statutory weight, regardless of the tire size; the 500 lbs. per inch width of tire IS NOT used, as tires not meeting the definition of “Wide-Based” do not fall under the 500 LB per inch width limitation.

When the temperatures cool down in the fall, it’s a good idea to check the air pressure in your tires. Tire pressure can drop dramatically with sudden drops in temperature. In general, for every 10-degree Fahrenheit change in air temperature, the pressure in a tire will change by one pound per square inch. Temperatures in Montana can suddenly drop by about 50 degrees in the fall which means the pressure in your tires can drop by 5 pounds per square inch.

Source: http://www.mdt.mt.gov/travinfo/tire_restrictions.shtml
Source: http://leg.mt.gov/bills/mca_toc/61_9_4.htm
Source: http://leg.mt.gov/bills/mca/61/9/61-9-406.htm