Winter Tires CanadaPlus sizing and minus sizing sound like opposites, and mechanically they are: both change the wheel diameter while holding the tire’s overall diameter nearly constant, by trading sidewall height for wheel diameter. Plus sizing goes up a wheel size for sharper response and looks, mostly a summer move. Minus sizing goes down a wheel size — and for winter it is the useful direction, because the tire that fits the smaller wheel is usually narrower, taller in the sidewall, and less expensive. Neither is improvisation. Vehicle manufacturers approve alternate sizes for winter use, and the approval only holds when the replacement matches the vehicle on the numbers that matter.
What plus and minus sizing actually change
A tire size such as 205/55R16 packs three numbers: section width in millimetres (205), aspect ratio — sidewall height as a percentage of width (55) — and wheel diameter in inches (16). Overall diameter is the figure the vehicle cares about: it sets gearing, speedometer reading and clearance. The formula is width multiplied by aspect ratio, doubled for the two sidewalls, converted to inches, plus the wheel diameter. Change the wheel by one inch and the aspect ratio must move the other way to compensate: a Plus One conversion pairs a wider, lower-profile tire with the bigger wheel; a Minus One conversion pairs a narrower, higher-profile tire with the smaller wheel. Transport Canada’s baseline still applies throughout: winter tires go on in sets of four, and handling is best when all four share the same type, size, speed rating and load index — a conversion changes all four corners together, never one axle at a time.
The diameter math, verified on a common size
Here is the check performed in full on one of the most common Canadian fitments, a 205/55R16. Sidewall height is 205 × 0.55 = 112.75 mm; doubled, that is 225.5 mm, or 8.88 inches; plus the 16-inch wheel gives an overall diameter of 24.88 inches. The two standard conversions:
| Fitment | Size | Overall diameter | Difference from stock |
|---|---|---|---|
| Stock | 205/55R16 | 24.88 in | — |
| Plus One | 225/45R17 | 24.97 in | +0.4% |
| Minus One (winter) | 195/65R15 | 24.98 in | +0.4% |
Both conversions land within about half a percent of stock, which is the point of the system: the speedometer, gearing and anti-lock braking calibration see effectively the same rolling diameter, while the contact patch shape changes completely. The Plus One tire is 20 mm wider with a much shorter sidewall. The Minus One tire is 10 mm narrower with a taller, more compliant sidewall — the winter-friendly trade. A candidate conversion that lands several percent away from stock diameter is not a conversion at all; it is the wrong tire, whatever the seller calls it. This verification is an original element of this guide on purpose: sizing advice that never shows the arithmetic asks for trust the reader should not have to give.
Why narrower is the winter direction
Continental’s tire-size guidance states the manufacturer position directly: different sizes are approved for winter use than for summer driving, and winter setups often feature a narrower width and a higher aspect ratio, which helps improve traction on snow and slush. The reasoning is contact pressure. The vehicle’s weight carried on a narrower patch presses the tread through loose snow and slush toward the firmer surface beneath, instead of floating on top of it. Transport Canada adds the mirror-image warning: wide, high-performance tires other than those designed as winter tires are not suitable for snow-covered roads. The taller sidewall of a minus-size fitment brings two practical bonuses — more compliance over frost heaves and broken pavement, and more rubber and air between a pothole edge and the wheel. The honest limits matter too: narrower is an advantage in snow and slush, not a universal upgrade, and dry-road cornering on a tall, narrow tire will feel softer than on the stock fitment. That trade is the winter bargain, taken knowingly.
Load index: the check that can veto a conversion
The service description after the size — for example 91H — carries the load index (91) and speed symbol (H). Load index is a code for the maximum weight one tire can carry when properly inflated, and the standard table is unforgiving: index 91 equals 1,356 lb per tire, 94 equals 1,477 lb, 95 equals 1,521 lb, 98 equals 1,653 lb and 102 equals 1,874 lb. The rule from sizing references is absolute: never install tires with a lower load capacity than originally specified. Conversions put this at risk because a shorter-sidewall or narrower tire in the same line can carry a lower index than the stock size — the diameter math can pass while the load math fails. Cold makes the margin thinner, not thicker: as Tire Warehouse’s winter guide notes, cold temperatures reduce tire pressure, and reduced pressure lowers load capacity. Trucks raise the stakes further, with indices such as 121 (3,197 lb per tire) and 126 (3,748 lb per tire) doing real work. If the candidate winter size shows a lower load index than the door-sticker specification, the conversion stops there — no traction argument overrides a capacity shortfall.
Speed rating: the winter exception, stated precisely
Two rules circulate and both are true, in different seasons. The general rule, from Continental’s plus-sizing bulletin, is to use replacement tires with an equal or greater speed rating than the original equipment tires. The same bulletin then states the winter exception: for driving in winter conditions it is generally acceptable to use winter tires with a lower speed rating than the original tires — however, doing so limits the maximum speed capability of the vehicle to the winter tire’s rating. Continental’s consumer guidance adds that this is permitted in many regions as long as the rating meets minimum legal requirements and the driver is informed, for example by a visible sticker. The common winter ratings are modest by design: Q is rated to 160 km/h, R to 170 km/h and T to 190 km/h, because the soft compound that stays flexible in the cold is also what limits sustained high-speed capability. A lower winter speed rating with a correct load index is normal and safe for cold-weather driving; the obligation it creates is honesty about the new ceiling, especially in spring, before the winter set comes off.
How to check your size before you buy
- Read the door sticker, not the old tire. The tire information label — usually on the driver’s door edge or door post — lists the manufacturer’s size, load index, speed rating and pressure for the vehicle. A previous owner’s fitment is not a specification.
- Confirm the alternate size is an approved one. The owner’s manual and manufacturer fitment guides list approved winter sizes for many models. An approved Minus One size settles the brake-clearance question at the same time — a smaller wheel must still clear the brake calipers, which is the most common reason a conversion is impossible on performance models.
- Run the diameter formula. Width × aspect ratio × 2, divided by 25.4, plus wheel diameter. The candidate should land within roughly 1 percent of the stock overall diameter; the worked example above shows the arithmetic.
- Compare the service description. Read the load index and speed symbol molded on the candidate tire’s sidewall before mounting. Load index must meet or exceed the sticker figure. A lower winter speed rating is acceptable under the exception — note the new maximum and respect it.
- Check the wheel, not just the tire. The tire must suit the wheel’s width and diameter, and the set of four must match. A minus-size winter package on its own wheels also keeps the summer wheels out of the salt.
- Set pressure to the placard figure for the vehicle, and recheck cold after the first sharp temperature drop, because pressure — and with it, load capacity — falls with the temperature.
What success looks like: all four winter tires carry a service description whose load index meets or beats the door sticker, the overall diameter computes within about 1 percent of stock, the wheels clear the brakes with no contact at full steering lock, and the speedometer reads normally against a GPS check on a straight, flat road.
Minus sizing is the rare modification that can improve winter traction and lower the bill at the same time — smaller winter tires and steel wheels usually cost less than the stock size. But it earns that verdict only after the load-index check. Diameter gets the attention; load capacity delivers the veto.
FAQ
Can any car minus-size for winter? No. The smaller wheel must clear the brakes, and an approved alternate size must exist. Some performance models cannot go smaller than the factory wheel. The owner’s manual fitment list is the authority.
Is a narrower tire always better in snow? It is the manufacturer-recommended direction for snow and slush because of contact pressure, and it is the standard winter conversion. It is not better at everything: dry handling softens, and the load index still has to match.
The winter tire’s speed rating is lower than the summer tire’s. Is that legal and safe? Under the published manufacturer guidance, yes for winter use, provided the rating meets minimum requirements and the driver treats that rating as the vehicle’s new maximum speed. The load index has no such exception.
Can the front axle be one size and the rear another? Not as a winter conversion. Transport Canada advises matched type, size, speed rating and load index on all four wheels, and warns against mixing sizes and constructions because it makes the vehicle less stable.