How to Read a Tire Sidewall

1. Width (millimetres)

The first number - 205 in 205/55R16 - is the tire's section width in millimetres: the distance across the tread from one sidewall's outer edge to the other's, measured on a specific manufacturer-approved rim width. A wider tire generally means more contact patch and grip, at the cost of some fuel efficiency and, on some vehicles, clearance.

2. Aspect Ratio (%)

The second number - 55 - is the aspect ratio: the tire's sidewall height as a percentage of its width. A 205/55R16 tire has a sidewall that's 55% of 205mm tall, or about 113mm. Lower numbers (a "lower profile") mean a shorter, stiffer sidewall - sharper handling, a firmer ride, and less cushioning for the wheel against potholes.

3. Construction

The letter right after the aspect ratio - almost always R on anything made in the last few decades - means radial construction: the internal fabric plies run radially (perpendicular to the direction of travel) rather than diagonally. Radial is the universal standard for passenger vehicles; the older diagonal/"bias-ply" construction (marked with a "D" or no letter at all) mostly survives on trailers and some specialty applications today.

4. Rim Diameter (inches)

The number after the construction letter - 16 - is the diameter, in inches, of the wheel/rim the tire is built to fit. This one has to match your wheel exactly; width and aspect ratio have some flexibility for a plus-size fitment, but rim diameter doesn't. Curious how a size swap affects your overall rolling diameter or speedometer accuracy? The tire size calculator works that out for you.

5. Load Index

The number just after the size (91 in 91H) is the load index - a code, not a direct weight, that maps to the maximum load in kilograms a single tire can carry at its rated pressure. Higher is stronger. Always match or exceed the load index your vehicle came with; going lower risks a tire that's not rated to safely carry your vehicle's actual weight, especially loaded up with passengers and cargo.

Standard load index reference chart. Always double-check the exact figure against your tire's own sidewall or the manufacturer - this is a quick reference, not a substitute for the number actually molded into your tire.

6. Speed Rating

The letter at the very end (H in 91H) is the speed rating - the maximum speed the tire is rated to sustain under its rated load. It's a safety margin, not a speed limit recommendation; as with load index, match or exceed what your vehicle originally came with.

Z is historically "over 240 km/h" and appears on a tire alongside a W or Y rating rather than by itself.

Season Markings

Tires signal what conditions they're built for with symbols, not the size code itself:

• M+S (or M&S) - "Mud and Snow." The baseline all-season marking; it doesn't guarantee serious winter performance on its own.
• Three-peak mountain snowflake (3PMSF) - certifies genuine severe-snow performance, tested against a real braking standard. Dedicated winter tires carry it, and so do newer "all-weather" tires, which pair it with the M+S marking to work year-round.
• No symbol - typically a summer/performance tire, not intended for winter conditions at all.

Quebec is the only Canadian province that legally requires winter or 3PMSF-rated all-weather tires on passenger vehicles, from December 1 to March 15.

Run-Flat & Extra Load (XL) Markings

Run-flat tires can be driven a limited distance (usually around 80km, at reduced speed) after losing pressure, thanks to a reinforced sidewall. There's no single universal marking for it - manufacturers use their own codes, like RFT, ROF, ZP, EMT, or SSR, so check your specific brand if you're not sure.

XL (Extra Load), sometimes marked Reinforced, means a tire built to carry more load at a given size than the standard version of that same size - common on heavier SUVs and vans.

DOT Date Code

Every tire has a DOT (Department of Transportation) serial number molded into one sidewall, ending in a 4-digit date code: the first two digits are the week of the year, the last two are the year. A code ending in 2223 means the tire was manufactured in the 22nd week of 2023. This matters even on unused tires - rubber ages and hardens over time regardless of tread wear, so a tire's age is worth checking alongside its remaining tread.

Tread Depth

Tread depth is usually measured in 32nds of an inch - a new passenger tire typically starts around 10-11/32". Most of Canada treats 2/32" as the legal minimum, though many mechanics recommend replacing winter tires closer to 5/32" once real snow and ice grip starts to suffer.

Every modern tire also has wear bars molded into the bottom of the tread grooves - small raised bridges that become flush with the tread surface once it's worn down to the legal minimum. If you can see a wear bar sitting level with the surrounding tread, it's time to replace. In a pinch, a Canadian quarter works too: insert it into a groove with the caribou upside-down - if you can see the top of its back, the tread's below 2/32" and needs replacing.