Montana Axle Weight Limits

Maximum Legal Weight Allowed

An axle may not carry a load in excess of 20,000 pounds, and no two consecutive axles more than 14 inches or less than 96 inches apart may carry a load in excess of 34,000 pounds. An axle 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. Axles 40 inches or less apart are considered to be a single axle. 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),

Where:

W = gross weight,
L = wheelbase in feet, and
N = number of axles, except 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 axle 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.

Notwithstanding a vehicle's conformance with the axle weight requirements, except for the steering axle, all axles weighing over 11,000 pounds must have 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. The maximum load on 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. 

Maximum Permit Weight Allowed

Source of information: http://www.mtrules.org/gateway/Subchapterhome.asp?scn=18%2E8%2E6
https://www.mdt.mt.gov/other/webdata/external/mcs/PERMIT-RESTRICTIONS.PDF