How to read a tire size

Every tire has its size moulded into the sidewall. Each part of the code means one thing. Here's a typical passenger car size, read left to right: P225/50R17 94V.

Reading P225/50R17 94V

1. P is the tire type: P-metric, a passenger car tire. LT means light truck. No letter usually means Euro-metric, also a passenger tire, built to a slightly different load standard.
2. 225 is the section width in millimetres, measured from sidewall to sidewall at the widest point.
3. 50 is the aspect ratio. The sidewall height is 50% of the width, so about 112 mm on this tire. A lower number means a shorter sidewall.
4. R is the construction. R means radial, which covers almost every car tire sold today.
5. 17 is the rim diameter in inches. This tire fits a 17-inch wheel. Width in millimetres, wheel in inches. Nobody ever fixed that.
6. 94 is the load index, a code for how much weight one tire can carry at its maximum pressure. 94 means 670 kg per tire.
7. V is the speed rating, the top speed the tire is rated to sustain at its rated load. V is 240 km/h.

Speed ratings

The speed rating comes from a lab test. Replacement tires should match or beat what your car calls for. The usual exception is winter tires, which are often rated lower than the factory tires. That's normal. Just remember the lower rating is now the limit.

Where to find your size

• On the sidewall of the tires on the car now.
• On the tire and loading information placard on the driver's door jamb. It lists the factory size and the recommended cold pressures.
• In the owner's manual. Some cars also have a sticker inside the fuel door or glovebox.

On a used car, check both. A previous owner may have changed wheel sizes, so the placard may not match what's on the car.

M+S and the three-peak mountain snowflake

M+S means mud and snow. It's based on tread geometry, mainly how much open groove space the tread has. There's no traction test behind it, and most all-season tires carry it.

The three-peak mountain snowflake (3PMSF) is a mountain outline with a snowflake inside. A tire only gets it after passing a standardized traction test on packed snow, measured against a reference tire. Winter tires and all-weather tires carry it. For a Toronto winter, this is the mark to look for.

The DOT date code

Near the rim you'll find a code that starts with DOT and ends in four digits. Those last four digits are the week and year the tire was made, so 2423 means the 24th week of 2023. Sometimes it's only on the inner sidewall.

Rubber ages even when the tread looks fine. Advice varies between tire makers, but most put an upper limit somewhere between 6 and 10 years from the date of manufacture. It's worth checking the date on any used tire before you buy it.

Why you shouldn't mix sizes

• All-wheel-drive systems are sensitive to differences in rolling diameter. Mismatched tires, even the same size with very different tread depths, keep the drivetrain working against itself and can damage it. Some AWD makers specify a maximum tread depth difference between tires.
• ABS, traction control and stability control compare wheel speeds. Different diameters can throw them off.
• Different sizes, tread patterns or constructions on the same axle can make the car pull or behave unpredictably in a hard stop.
• Some cars run wider tires on the rear from the factory. That's called a staggered setup, it's by design, and the placard lists both sizes.

Plus-sizing and minus-sizing

Plus-sizing means a bigger wheel with a lower-profile tire, keeping the overall diameter close to the original. P225/50R17 is about 657 mm tall. A 225/45R18 on an 18-inch wheel comes out to about 660 mm. Close enough.

• Keep the overall diameter within about 3% of the original.
• The load index has to be equal or higher. Lower-profile sizes often need an XL (extra load) version to match.
• Check clearance at full steering lock and with the suspension compressed, not just parked.
• A shorter sidewall means a firmer ride and more risk of pothole damage.

Minus-sizing goes the other way and is common for winter tires: a smaller wheel with a taller, often narrower tire. For the example size, 215/60R16 comes out to about 664 mm, close to the original 657, as long as a 16-inch wheel clears the brake calipers. Not every car allows it, so we check before ordering.