Winter Tires CanadaStart with the misconception: four tires with the same size written on the sidewall are not necessarily four matched tires. A tire from a different brand can measure differently in actual diameter from another tire carrying the identical size marking — brands run large or small against the labelled size. A new tire and a worn tire of the same model differ too, because tread depth is part of the diameter. On a two-wheel-drive vehicle a modest difference is mostly a handling note. On all-wheel drive it becomes a mechanical load, applied every kilometre, in exactly the season when the vehicle is working hardest.
What the drivetrain actually measures: rolling circumference
An AWD driveline mechanically connects the tires so they work in unison. A transfer case distributes power and torque to both the front and rear axles, and the additional differentials and viscous couplings in the system are designed to allow momentary wheel-speed differences — a corner, a tire spinning briefly on ice. A circumference mismatch is not momentary. It is a permanent instruction that one wheel must turn faster than the others to cover the same ground, and components forced to process tires rolling at different speeds continuously face high temperatures and premature wear. The failure mode is not drama; it is heat and wear accumulating in parts that were built to absorb seconds of difference, not seasons of it.
The differences do not need to come from a wrong purchase. Tire Rack’s matching guidance lists the everyday causes: different sized tires, different tread designs, tires from different manufacturers, different inflation pressures, and tires worn to different tread depths. Its definition of matched is correspondingly plain — all four tires the same brand, the same design, and the same tread depth — and its warning is that mixing brands, designs and depths may cause components in the driveline to fail.
The tolerances are vehicle-specific, not universal
Here the common advice needs correcting, because the figure most often quoted as an industry standard is not one. Published manufacturer guidance disagrees with itself, which is the clearest sign that the manual, not the internet, holds the number for a given vehicle:
| Source | Published matching figure | Form of the figure |
|---|---|---|
| Audi (owner’s manual, via Tire Rack) | Rolling radius of all four tires must remain the same, or within 4/32 in of each other in remaining tread depth | Tread-depth difference |
| Subaru (via Tire Rack) | Circumference difference no more than 1/4 in (6 mm) | Circumference difference |
| Subaru (owner’s manual) | Same size, circumference, construction, brand and load range, with the same amount of wear on all four — no numeric tolerance published | Specification, no number |
| Michelin (when the manual gives no mixing instructions) | All four the same size, no mixing radial with bias-ply, outside circumferences within 1 in (2.5 cm) of each other | Circumference difference |
| Porsche Cayenne (via Tire Rack) | Within 30% of the other tire on the same axle’s remaining treadwear | Same-axle treadwear proportion |
| Nissan GT-R (via Tire Rack) | When replacing fewer than four, each tire staying in service must have at least 6/32 in (5 mm) of remaining tread depth | Minimum remaining depth |
Read the table as a spec-verification exercise. The figures use three different measures — tread depth, circumference, and proportion of remaining wear — and they do not convert into one another neatly, because tread depth lost at the tread is only part of a tire’s circumference. A single figure quoted without a vehicle attached, however confident it sounds, cannot be checked against a manual that may not contain it. Honda’s manual language makes the same point from the other side: same size, load range, speed rating and maximum pressure, replace all four at once or otherwise in front/rear pairs — again a specification for that vehicle, not a universal constant.
The winter half of the rule: grip must match too
Almost all of that debate is about diameter. Winter adds a second mismatch that diameter never captures. Transport Canada’s winter guidance is explicit: install winter tires in sets of four, with the same type, size, speed rating and load index on all four wheels, and do not mix tires with different tread patterns, internal construction and size, because mixed tires make the vehicle less stable. Two winter tires on one axle and two all-season or summer tires on the other can be perfectly matched in circumference and badly mismatched in grip — and on ice the grip difference arrives first, in braking and in a turn, long before the drivetrain notices anything. The changeover bargain of fitting only two winter tires fails both halves of the AWD rule at once. Four winter tires, of one model, is not thoroughness; it is the specification.
The symbol to verify on each of the four is the peaked mountain with the snowflake: tires marked with it meet specific snow traction performance requirements and were designed for severe snow conditions. And a matched set still has a wear floor — Transport Canada advises not using tires worn close to 4 mm (5/32 in) of tread depth on snow-covered roads, however evenly the four have worn.
One damaged winter tire: the measure-then-decide procedure
Mid-season, one tire is damaged beyond repair and the other three have real wear on them. This is a decision procedure, not a guess:
- Measure all four positions. Gauge the remaining tread depth of the three surviving tires and note each reading; the deepest and shallowest readings set the spread the replacement has to fit.
- Find the vehicle’s own figure. Read the owner’s manual for its matching requirement — a tread-depth difference, a circumference difference, or a specification with no number. That document, not a general guide, decides.
- Price the three honest options. A single new tire of the same brand and model, shaved by a shop equipped to do it down to the tread depth of the others; a matched pair; or a full set of four. Shaving exists precisely for the nearly-new set with one casualty — it discards tread the buyer just paid for, which is why it suits a young set and not a worn one.
- Check the survivors against the winter floor as well as the tolerance. If the three are already close to 4 mm of tread, matching a new tire to them solves the driveline question and leaves the snow question unanswered; a set of four is the cleaner call.
- Record what was fitted, where, and at what depth. One line per position, dated. The next rotation or repair starts from that record instead of from memory.
What success looks like: all four corners carry the same winter tire model, the measured tread-depth spread sits inside the figure in the vehicle’s manual, and the shallowest tire is still clear of the 4 mm snow floor — verified with a gauge, not estimated from the driver’s seat.
Rotation: how a matched set stays matched
Matching decays. Front and rear tires wear at different rates even though all four are driven, so a set fitted perfectly in November drifts apart unless it is rotated. Frequent rotation is what keeps the tread-depth difference — and therefore the circumference difference — from growing between the axles. Treat rotation as part of the matching specification rather than a separate maintenance nicety: it is the step that makes “all four must match” remain true in February, at the pressures the placard specifies, on the set that was matched in the fall.
— Owen Park, Automotive Writer