Ask any tire shop for a quote and the first thing they'll want is your tire size. It's moulded into the sidewall in raised letters, usually something like 205/55R16 91H. Every part of that string means something specific, and a couple of the parts people skip over matter more than they think.
Where to find it
Two places. The sidewall of the tire itself, and the tire placard, a sticker on the driver's door jamb. On some cars it's on the fuel door or the glovebox lid instead. The placard lists the size the car came with from the factory and the recommended cold tire pressures in kPa and psi.
If the tires on the car don't match the placard, someone changed them along the way. That isn't necessarily wrong, but it's worth knowing before you order a new set.
The size, piece by piece
Using 205/55R16 91H as the example:
| Part | Example | What it means |
|---|---|---|
| Width | 205 | Section width in millimetres, measured across the widest point of the tire |
| Aspect ratio | 55 | Sidewall height as a percentage of the width. Here, 55% of 205 mm, or about 113 mm |
| Construction | R | Radial. Nearly every passenger tire sold today is a radial |
| Rim diameter | 16 | Wheel size in inches. The width is metric and the rim is imperial, and nobody is going to fix that now |
| Load index | 91 | How much weight each tire can carry at its rated pressure. 91 is 615 kg |
| Speed rating | H | The highest sustained speed the tire is rated for. H is 210 km/h |
Some tires have a letter in front of the size. P (as in P205/55R16) means a P-metric passenger tire. LT means light truck, a heavier construction for pickups and vans that haul or tow. No letter usually means a Euro-metric tire. P-metric and Euro-metric tires of the same size can have different load ratings, so check before swapping one for the other on a heavy vehicle.
Load index and speed rating
These are the two numbers people ignore. The load index tells you how much each tire can carry. A replacement tire should have a load index at least as high as what's on the placard. Going lower on an SUV, a minivan full of people or a pickup that tows is how tires overheat and fail early.
Common speed ratings:
- S: 180 km/h
- T: 190 km/h
- H: 210 km/h
- V: 240 km/h
- W: 270 km/h
- Y: 300 km/h
Nobody is doing 210 on the 401 legally. The rating still matters, because a higher-rated tire is generally built to handle heat better and often steers more precisely. For all-season and summer tires, stick with the original rating or higher. Winter tires are often sold with a lower speed rating than the factory tires. That's common practice, but the load index should still meet the placard.
Other markings worth knowing
- XL or Extra Load: a reinforced tire that carries more weight at a higher inflation pressure. Common on SUVs and some EVs.
- M+S: mud and snow. Most all-season tires have it. It is not a winter rating.
- Three-peak mountain snowflake: the tire met a standard snow traction test. Winter and all-weather tires carry it.
- Max. press.: the most pressure the tire can safely hold. It's not what you should inflate it to. Use the door placard for that.
- Treadwear, traction and temperature grades: a US rating system that most tires sold in Canada also carry. Treadwear numbers are only useful for comparing tires from the same manufacturer.
The date code
Near the rim you'll find a code that starts with DOT. The last four digits are the week and year the tire was made, so 2324 means the 23rd week of 2024. Tires age even when they're sitting in a warehouse or a garage, so check the code when you buy and look at it on older sets. Many tire makers recommend a yearly inspection once a tire reaches five years old and replacement by ten years regardless of tread. Your tire maker's guidance may differ, so check it.
Changing sizes
Running a different size from the placard isn't automatically a problem. Going down one rim size for winter tires is common, because a smaller wheel with a taller sidewall usually costs less and takes potholes better. The overall diameter needs to stay close to the original, though, or the speedometer and odometer will read off and some driver-assist systems may not behave as designed. The smaller wheel also has to clear the brake calipers.
Before you call around for prices, take a photo of the sidewall and one of the door placard. It saves a lot of back and forth, and it means the quote you get is for the tire that actually fits your car.
