Every winter tire carries its whole specification on the sidewall, in a string like P225/65R17 102T that most buyers never decode. That is a problem in October, when the decision that actually matters is not the brand on the banner but whether the tire in front of you matches your vehicle's size, carries enough load, and holds an adequate speed rating for winter use. This guide decodes the string piece by piece with a worked example, then covers the two winter-specific markings — the three-peak mountain snowflake and the DOT date code — that decide whether a tire belongs on a Canadian car at all.
| Element | Meaning | In the example |
|---|---|---|
| P | Service type: P = passenger vehicle; LT = light truck; T = temporary spare; ST = special trailer | Passenger tire — cars, minivans, SUVs; P-metric is the most common fitment in Canada |
| 225 | Section width, sidewall to sidewall, in millimetres | 225 mm — common Canadian sizes run 205 to 245 |
| 65 | Aspect ratio: sidewall height as a percentage of width | 65% of 225 mm = about 146 mm of sidewall; higher number, smoother ride; lower number, sharper handling |
| R | Radial construction | Radial — used in roughly 99% of modern tires |
| 17 | Rim diameter the tire fits, in inches | 17-inch wheel; common Canadian sizes run 15 to 20 inches |
| 102 | Load index: maximum load per tire at proper inflation | 850 kg per tire — 3,400 kg across four tires |
| T | Speed rating: maximum sustained speed capability | 190 km/h; S = 180, H = 210, V = 240 |
The verification habit is simple: read the string off your current tires (or the door-jamb placard, which states the manufacturer's specified size), and require any winter replacement to match the size elements exactly and meet or exceed the load index. Sellers make honest mistakes and listings carry typos; the sidewall is the document that counts.
The load index is a code, not a weight — you look it up. Reference points worth memorizing: 91 = 615 kg, 95 = 690 kg, 100 = 800 kg, 102 = 850 kg, 104 = 900 kg per tire. Two winter-specific cautions come from truck-tire guidance. First, choose a load index equal to or greater than the vehicle's requirement — never lower; the placard and owner's manual state the requirement. Second, cold works against you: low temperatures drop tire pressure, and underinflation lowers effective load capacity, so a tire with a modest buffer above the minimum handles a cold snap more forgivingly than one at the exact floor. For trucks the scale runs higher — load index 121 supports about 3,197 lb per tire and 126 about 3,748 lb — and winter loads (a plow, a full bed, towing) eat the margin faster than summer loads do.
Winter tires commonly carry Q (160 km/h), R (170 km/h) or T (190 km/h) ratings — lower than the H or V on many summer tires. That is not a defect. Winter compounds are softer so the rubber stays flexible in the cold, and that softness limits high-speed capability; the trade is deliberate, and the ratings remain far above any legal or sensible winter driving speed. The practical rules: never fit a tire whose ratings fall below the vehicle manufacturer's specification without confirming the exception — many manufacturers explicitly permit a lower speed rating for dedicated winter tires provided the size and load requirements are met, and the driver's manual is the authority — and remember the rating assumes proper inflation and load, both of which winter undermines if you let them slide.
The three-peak mountain snowflake (3PMSF). Beyond the size string, a true winter tire carries the 3PMSF symbol, which certifies the tire met a snow-traction performance standard rather than merely being marketed for the season. A tire lettered M+S without the mountain snowflake is an all-season with a mud-and-snow tread claim — a different thing, and the distinction is the single most common buying mistake in this category. Check for the symbol physically on the sidewall of every tire in the set, including used sets, where one mismatched replacement tire is a classic find.
The DOT date code. The last four digits of the DOT code give the week and year of manufacture — 0923 means the ninth week of 2023. Age matters because rubber hardens over time: the common guidance is that tires older than about six years should usually be replaced regardless of remaining tread. A “new” winter set that has sat in a warehouse for three years has already spent half that allowance before its first season, which is why the date code belongs in every price negotiation and every used-set inspection.
Beyond the main string, sidewalls carry secondary markings worth recognizing at the counter. XL (extra load) denotes a reinforced tire built to carry a higher load at higher inflation than the standard tire of the same size — relevant when a vehicle's placard specifies XL fitments, as many crossovers now do. Run-flat markings (each manufacturer uses its own code) identify tires designed to be driven a limited distance after a puncture; they must be matched as a set and paired with a working TPMS, because the stiff sidewall hides the pressure loss a driver would otherwise feel. Rotation arrows on directional winter tires are fitment instructions, not decoration: the tire only delivers its designed snow and slush evacuation rolling the marked way, which is why the changeover checklist includes verifying all four arrows. None of these markings change the core decode — size, load, speed — but each one is a reason a tire that matches on the main string can still be the wrong tire for a specific vehicle. When a secondary marking on the candidate set differs from the set coming off, that difference belongs in the conversation with the installer before the tires are mounted, not after.
Can I downsize my winter tires to a narrower size? Minus-sizing is a long-standing practice — a narrower tire on a smaller rim can improve snow bite and cost less — but it changes the size string legitimately, so it must be done from the manufacturer's approved alternate sizes, keeping load index at or above spec. It is a fitment decision for a tire professional, not a guess from a listing.
Is a higher load index always better? No. Equal-or-greater is the requirement; far higher usually means a stiffer, heavier tire (often an LT tire) with a harsher ride on a vehicle that never needed it. Match the requirement with a small buffer, not a maximum.
What if one tire in a used set has a different code? Treat it as a three-tire set plus a spare-grade extra. Mismatched size or load across an axle undermines handling predictability in exactly the conditions winter tires exist for.
Winter Tires Canada