3PMSF vs M+S: what the snow symbols mean

M+S (also written M&S, M/S or M-S) stands for mud and snow. Under the U.S. Tire Manufacturers Association’s definition, a passenger or light-truck tire may carry it if its tread meets two geometric criteria, both measured from the dimensions of the mold:

That’s all. The definition requires no traction measurement, no particular rubber compound and nothing about cold-weather performance. Continental says plainly that M+S performance is neither defined nor measured. Trade press traces the mark to the 1970s, when it was a way to identify aggressive tread patterns. Today it appears on most all-season tires.

The three-peak mountain snowflake, abbreviated 3PMSF and called the “Alpine symbol” in Europe, was defined in 1999 by the U.S. and Canadian tire industries for tires meant for severe snow. Unlike M+S, it is earned in a test.

The test is ASTM F1805. An instrumented vehicle accelerates in a straight line on medium-packed snow while it measures the driving traction of the tire being tested, and the result is compared with a Standard Reference Test Tire run under the same conditions. To qualify, the tire must reach a snow traction index of at least 1.10 against the older P195/75R14 reference tire, or 1.12 against the newer P225/60R16 reference that replaced it after the old one went out of production in 2020. In plain terms, the tire has to deliver at least 10 to 12% more snow traction than a standard all-season reference tire. Light-truck tires face the same threshold.

The same definition is written into U.S. and Canadian federal tire regulations, and FMVSS No. 139 lets a qualifying tire display the symbol at the manufacturer’s option. The symbol has fixed proportions: a mountain at least 15 mm wide and tall, with three peaks, the middle one tallest, and a six-sided snowflake inside.

The snowflake is a meaningful mark, but it is narrower than many shoppers assume. ASTM’s own description of F1805 limits it to straight-line driving traction. Braking and cornering on snow are measured with a different method that the North American definition doesn’t use.

No U.S. or Canadian regulation defines “all-season” or “all-weather”, so the sidewall symbols are the only standardized signal. Even the labels don’t line up: Michelin and Goodyear sell 3PMSF-marked tires as all-season tires, while Nokian, Toyo and Bridgestone call similar tires all-weather. Here is how the categories usually sort out by symbol:

The important point is that an all-weather tire and a dedicated winter tire can carry exactly the same symbol and still be built for different jobs. Michelin positions its CrossClimate2 for year-round driving with occasional winter conditions and points drivers who face frequent snow and ice to its winter tires. Nokian says that in extreme winter conditions, nothing is safer than a dedicated winter tire. For more on choosing between them, see our winter tire guide.

Most of the U.S. has no winter tire requirement, but some places do, and they draw the line in different spots:

Québec is the only Canadian province with a province-wide mandate. Since December 15, 2014, only tires with the mountain-snowflake pictogram, or studded tires, count as winter tires there. A 2019 change moved the start of the season from December 15 to December 1, and it still ends March 15. All-weather tires with the symbol are legal.

British Columbia requires winter tires or chains on signed highways, and M+S is the legal minimum, with at least 3.5 mm of tread. The province recommends mountain-snowflake tires for anyone who drives those highways regularly, and says chains over summer tires don’t replace legal winter tires.

Colorado’s traction law accepts ordinary M+S tires. A 2026 law, HB26-1237, removed the old condition that the vehicle be four- or all-wheel drive, so from August 12, 2026 any vehicle can comply with tires that have at least 3/16 inch of tread and an M+S, M/S or mountain-snowflake marking, or an all-weather rating from the manufacturer. Chains or an approved traction device also comply. CDOT still recommends four- or all-wheel drive in the mountains.

Germany shows where Europe has gone: tires driven in wintry conditions must carry the Alpine symbol, and the grace period for older M+S-only tires ended on October 1, 2024.

Transport Canada, Québec’s SAAQ, Michelin and Nokian all use about 7°C (45°F) as the point where all-season and summer rubber starts to stiffen and lose grip, and where winter tires start to pay off. It is useful guidance for timing a seasonal swap. It isn’t an engineering cutoff, though: none of the test standards or regulations behind the snow symbols mentions it, and how a compound behaves in the cold varies from tire to tire.

Whatever you fit, Transport Canada recommends winter tires in sets of four, not mixed with other tread patterns or sizes. And check pressure when the weather turns: cold air lowers tire pressure by about 1 psi for every 10°F, as our tire pressure guide explains.