Winter tires: look for the mountain snowflake, not just M+S, and put them on all four wheels

M+S vs the mountain snowflake

• M+S means limited. USTMA says the M and S letters, also written MS, M/S or M&S, mean a tire is intended for limited mud and snow service. The mark is common on all-season tires as well as winter tires. [S2]
• The snowflake means tested. To carry the mountain snowflake, a tire has to reach a snow grip index of at least 1.10 against a standard reference test tire (or 1.12 against a newer one) in a straight-line driving traction test on medium-packed snow. In plain terms, at least 10 or 12 percent more grip than the reference tire in that test. [S1] [S5]
• What to look for. A mountain with three peaks, the middle one tallest, with a six-sided snowflake inside, next to the M+S letters. USTMA sets a minimum size of 15 mm wide and 15 mm tall. Canada’s regulations call it the Alpine Symbol. [S1] [S5]
• Why it’s different. USTMA says tires built for severe snow generally have tread patterns, structure and materials that do better in snow than tires meeting its basic snow tire definition. Transport Canada says to look for the symbol when shopping for winter tires. [S1] [S4]
• What the test doesn’t cover. The test measures driving traction in a straight line on packed snow. Neither USTMA nor Canada’s rule says it measures ice braking or cornering, so drive for the conditions either way. [S1] [S5]

Why the cold matters, not just the snow

• The 7°C cue. Transport Canada says that below 7°C (about 45°F), all-season and summer tires begin to lose elasticity, which means less traction. Winter tires keep their elasticity and grip at much lower temperatures. [S4]
• Wide performance tires. Transport Canada adds that wide, high-performance tires that aren’t made as winter tires aren’t suitable for snow-covered roads. [S4]
• NHTSA’s nudge. NHTSA’s winter tips say to consider installing snow tires as part of getting the car ready. [S9]
• Packing the car too. Our winter car kit guide covers what to keep in the trunk.

Put them on all four wheels

• Never just the front. USTMA says if winter tires go on the front axle, they must go on the rear too. That applies to front-wheel-drive, 4x4 and all-wheel-drive vehicles alike. Winter tires only on the front can make the car handle badly, and USTMA warns that can lead to a crash. [S2] [S3]
• All four is the goal. USTMA says it’s always preferable to fit winter tires to every wheel position. Transport Canada says to install them in sets of four. [S3] [S4]
• Why the rear matters. Getting moving depends on the tires on the drive axle, but USTMA says handling, cornering and braking depend on the front and rear tires, and the rear tires need at least as much traction as the front. [S3]
• Match them. Transport Canada says handling improves with the same type, size, speed rating and load index on all four wheels. Don’t mix tread patterns, internal construction or sizes. Put the tires with the deepest tread on the rear. [S4]
• Speed rating. USTMA says a winter tire with a lower speed rating than the original tire is generally acceptable, but then the vehicle’s speed has to stay within the winter tire’s rating. Check your owner’s manual for any limits. [S2] [S3]

Tread and pressure in the cold

The numbers below come from different sources for different jobs. The lowest is a replacement point, not a snow target. [S4] [S6] [S7] [S9]

• Measure before the first storm. Our tire tread guide walks through the penny test and the wear bars.
• Pressure drops with the temperature. USTMA says tires lose about 1 to 2 psi for every 10°F drop. A tire that reads right in a warm garage will be low outside when it’s below freezing, Transport Canada says. [S2] [S4]
• Check cold, fill to the label. NHTSA says to check tires when they haven’t been driven for at least three hours and to fill them to the vehicle maker’s pressure on the driver’s door frame label, not the number on the tire. Our tire pressure guide shows where to find it. [S9]
• Don’t let air out for grip. USTMA says lowering pressure to get more traction on snow or ice doesn’t work and leaves tires open to damage from underinflation. [S2]

Studs, chains and state rules

• Washington stud dates. Studded tires are legal in Washington only from November 1 through March 31. That applies to every vehicle in the state, including ones from other states, with no exemptions or waivers. [S6]
• Studs aren’t chains. WSDOT says studded tires don’t count as chains when chains are required. Stud-free winter traction tires are legal year-round there. [S6]
• Other states vary. USTMA says some states ban studded tires and many limit when you can use them. Check your state’s rules before you buy. [S2] [S3]
• Studs go on new tires, all four. USTMA says only new tires should get studs, never used ones, and studded tires on the front must also go on the rear. It strongly recommends all four. [S3]
• Colorado’s traction law. When it’s on, passenger vehicles need tires with 3/16 inch of tread marked with the mountain snowflake, M+S or M/S, or chains or an approved traction device. CDOT says it can be enforced on I-70 from Morrison to Dotsero between September 1 and May 31, and on any state highway when conditions call for it. [S7]
• California chain controls. Caltrans uses three levels. R1 lets cars and light trucks under 6,000 pounds skip chains with snow tires on at least two drive wheels. R2 lets four-wheel and all-wheel drive vehicles skip chains with snow-tread tires on all four wheels. R3 means chains on everyone. Vehicles using the exemptions still have to carry chains or traction devices. [S8]
• Carry chains for the mountains. WSDOT says anyone crossing the mountains should carry chains and practice putting them on before leaving home. [S6]

Quick decision

Pick what you have and where you drive to see what the sources say in one place. [S2] [S3] [S4] [S6] [S7] [S8]

Your situation

Pick the one that fits to highlight what the sources say.

Before the first snow

Quick answers