3PMSF vs M+S: what winter tire markings mean

3PMSF vs M+S: the federal snow traction test behind the mountain snowflake symbol, what M+S does and doesn't promise, and what to tell a customer.

3PMSF vs M+S, the short answer

The mountain snowflake symbol, often called 3PMSF, means the tire passed a standardized snow traction test. M+S means the maker designed the tire for mud and snow by its own criteria, and no standardized winter test stands behind those letters.

That's the difference. Under the federal tire standard for light vehicles, only the first one counts as a "snow tire."

M+S shows up on most all-season tires, per Michelin. So M+S with no mountain snowflake on a customer's car usually means all-seasons, not winter tires.

What the mountain snowflake means under 49 CFR 571.139

Federal Motor Vehicle Safety Standard 139 covers new radial tires for vehicles with a gross vehicle weight rating of 10,000 pounds or less. Its definitions section, S3, says a snow tire is one that reaches a traction index of 112 or higher compared with the ASTM F2493 standard reference test tire, using the snow traction test on a medium-pack snow surface described in ASTM F1805-20, and that is marked with the Alpine symbol on at least one sidewall.

Both halves matter. Pass the test, carry the mark.

The 112 is a comparison, not a grip reading on its own. It's measured against a reference tire, and the definition names both pieces: the ASTM F2493 reference tire and the snow test described in ASTM F1805-20.

The definition also asks for the symbol on at least one sidewall, not both. If you don't see it on the side facing you, turn the tire around and look at the other side before you call it M+S only.

S5.5(i) describes the symbol. It's optional: a tire that meets the snow tire definition "may, at the option of the manufacturer," show the pictograph of a mountain with a snowflake. If the maker uses it, the mountain needs a base of at least 15 mm and a height of at least 15 mm, with three peaks and the middle one tallest. Inside sits a six-sided snowflake at least half as tall as the tallest peak.

The standard treats snow tires a little differently in testing, too. Its endurance test runs at not less than 120 km/h, and its low-pressure test at 120 km/h, but for snow tires both run at not less than 110 km/h.

One scope note. FMVSS 139 doesn't apply to light truck tires with a tread depth of 18/32 inch or more, among other exclusions. That means 571.139's snow-tire definition doesn't apply to those tires. Keep it in mind before you talk up a deep-tread LT tire's snow rating.

What M+S means, and what it doesn't

Michelin's tire markings page puts it in one line: M+S "indicates the tire is designed for mud and snow conditions based on the manufacturer's criteria, but does not require standardized winter performance testing." Its FAQ adds that the marking is based on the maker's own tread geometry criteria. And it closes with the point a counter worker should repeat to customers: only tires carrying the 3PMSF symbol have verified winter performance.

None of that makes M+S meaningless. NHTSA describes all-season tires as handling a variety of road conditions with "some mud and snow capabilities." That's a fair way to describe an M+S all-season. Some, not a lot.

What M+S doesn't do is prove anything about snow traction. No federal test stands behind it. Don't sell it as a winter tire.

Winter, all-season, summer: what NHTSA says

NHTSA's tire buying guidance starts with geography: the right type "will vary by where you live and the typical weather you drive in." Here's how it describes each type.

NHTSA's winter driving page goes one step further and tells drivers to consider installing snow tires. It also says to check tire safety ratings before buying new ones.

What to tell a customer at the counter

Start with where they drive. A customer who sees real snow needs a different answer from one who sees a dusting twice a year, and NHTSA's own buying advice hinges on that question.

Then show them the sidewall. Point to the mountain snowflake if it's there, and to the M+S letters if that's all there is. Say it plainly: the mountain means it passed a snow test; M+S means the maker says it's built for mud and snow.

Check pressure. NHTSA's winter page says inflation pressure drops as the outside temperature drops, and that each tire should be filled to the vehicle manufacturer's recommended pressure, found in the owner's manual and on the driver's door frame label. Not the number on the tire. That one is the maximum the tire can hold.

Check tread and age. The same NHTSA page says tread should be at least 2/32 inch on all tires, and notes that some vehicle manufacturers recommend replacing tires every six years regardless of use. The tread depth chart has the readings, and the DOT date code guide shows how to read a tire's age off the sidewall.

This guide covers the federal U.S. standard only. It makes no claim about any state, province or country that sets its own winter tire rules.

Grading used winter tires

When winter tires come in as used stock, grade them like any other tire, with one more field. Write down which mark the tire carries: mountain snowflake, M+S only, or both. Under the federal definition, only the first kind is a "snow tire."

Age matters here. NHTSA lists infrequent use and poor storage among the things that add to tire aging, and a winter tire spends months in storage. The tire age guide covers when to turn one down, and the used tire grading checklist covers the rest of the inspection. In DuroBay, each tire has notes, so "3PMSF" or "M+S only" can go right on the record next to the size, brand, model and DOT that the sidewall photo fills in for you to confirm.

If a customer is swapping between a winter set and an all-season set at the same visit, it's a good moment to check the rotation pattern too. The tire rotation guide covers which way each setup can move.