M+S (mud and snow)

How to read it

USTMA’s Tire Information Service Bulletin 10 gives the criteria for marking the letters M and S (as M+S, M.S., M&S, M-S or M/S) on a passenger or light truck tire. At least one tread edge needs pockets or slots reaching at least 12.7 mm (1/2 in) toward the tread centre, at least 1.6 mm (1/16 in) wide, with edges at 35 to 90 degrees to the direction of travel, and the tread’s contact surface must be at least 25% void, all measured on the mould. USTMA’s guide says the letters mean the tire is intended for limited mud and snow service, and that the mark is common on all-season and winter tires. Bridgestone’s glossary reads M+S as meeting the USTMA/RAC definition of an all-season tire. NHTSA wrote in 1986 that it had issued no specification for mud and snow tread design.

Also searched as: M+S, M/S, MS, mud and snow.

M+S or the mountain snowflake?

The three-peak mountain snowflake is earned in a snow traction test; M+S is not. USTMA’s severe snow definition (TISB 37) requires a measured snow grip index, and marks the tires that reach it with the M and S letters plus the snowflake pictograph, so under that definition a severe snow tire carries both; British Columbia notes that some makers mark both. Continental calls the snowflake a more stringent certification, and says a tire with it has to prove itself in a standardized brake test on snow, which the M+S marking did not require.

Differences by country or standard

In British Columbia, the letters M and S are the minimum for a legal winter tire on signed highways, with at least 3.5 mm of tread. TranBC, the province’s transportation ministry, says the tire industry indicates M+S tires have a hard compound with reduced traction below 7 °C (45 °F) compared with tires carrying the mountain snowflake. Pirelli’s UK guide says M+S shows some ability in mud and snow, and that a true winter tyre also has the mountain symbol.

Why an M+S tire still has UTQG grades

The US quality grades exempt deep tread, winter-type snow tires. NHTSA explained in 1986 that all-season tires, designed for use throughout the year, don’t qualify for that exemption, so an M+S all-season tire still carries a treadwear grade.