How to Read a Tire Size | PTP's Lift & Fix

Stamped on the sidewall of every tire is a string of numbers and letters that looks like a secret code. Something like 225/65R17 102H. It's not secret, it's just written for engineers, not humans. Once you know how to read it, buying tires stops feeling like a guessing game.

You need this when you're shopping online, comparing winter vs all-season options, or checking whether a tire your buddy is selling actually fits your car. Five minutes of learning saves you from buying the wrong size and finding out in the parking lot.

Where to find your tire size

Look on the sidewall of the tire itself, the raised letters facing outward. You'll also find the recommended size on a sticker inside the driver's door jamb, and in the owner's manual. The door sticker is the best reference because it lists what the car was designed for, including load and pressure info.

If your tires don't match the door sticker, someone changed sizes before you owned the car. Figure out whether that was done properly before you buy replacements "the same as what's on there now."

Breaking down 225/65R17 102H

Let's walk through each piece using that common example.

225 section width (millimetres)

The first number is the width of the tire tread from sidewall to sidewall, measured in millimetres. 225 means 225 mm wide. Wider tires can mean more grip but also more road noise and fuel use. Narrower tires can cut through snow differently. Stick close to factory width unless you know what you're changing and why.

65 aspect ratio (profile height)

The second number is a percentage, not millimetres. It's the height of the sidewall as a percent of the width. So 65 means the sidewall height is 65% of 225 mm, which works out to about 146 mm.

Lower numbers (50, 45) mean shorter sidewalls, "low profile" tires. They look sporty and handle sharp but ride harsher and are easier to damage on potholes. Higher numbers (65, 70) mean taller sidewalls softer ride, more flex, common on SUVs and trucks.

R construction type

Almost every passenger tire today is R for radial. You might see D (diagonal/bias) on some trailers or old equipment. For your daily driver, it's R. Don't overthink this one.

17 wheel diameter (inches)

This is the size of the wheel the tire mounts on, in inches. A 225/65R17 fits a 17-inch wheel. This number must match your rim. You can't put a 17-inch tire on a 16-inch wheel physics says no.

102 load index

The load index is a code for how much weight one tire can support at max pressure. 102 translates to about 850 kg per tire. You can look up load index charts online, they're standardized.

Match or exceed the load index on your door sticker. Going lower is unsafe and illegal in many places. Going slightly higher is usually fine.

H speed rating

The letter at the end is how fast the tire is rated to run safely for sustained periods. H = up to 210 km/h. Common ratings you'll see:

S or T: everyday passenger tires, lower speed cap
H or V: common on sedans and crossovers
W, Y, Z: performance cars

For normal driving in Ontario, anything at or above your factory rating works. Don't downgrade below what the manufacturer spec'd.

Plus-sizing: when numbers change on purpose

Some owners upsize wheels for looks bigger diameter rim, lower aspect ratio, keeping overall diameter roughly the same so the speedometer stays accurate. That's called plus-sizing. Done wrong, you get rubbing on fenders, wrong speedo readings, and traction control acting weird.

If you're not deliberately plus-sizing, buy the size on the door sticker. Full stop.

Overall diameter: why it matters

Your car's gearing, ABS, and speedometer assume a certain total tire height width, sidewall, and wheel combined. Change overall diameter too much and:

Speedometer reads wrong
Odometer accumulates error
Clearance changes might rub on turns or over bumps
All-wheel-drive systems can bind if front and rear diameters don't match

Tire shop websites often show overall diameter when you compare sizes. If you're within a few millimetres of stock, you're usually okay. Big swings need research.

Other markings worth knowing

Beyond the main size string, the sidewall has more info:

M+S or mountain/snowflake symbol: mud and snow tread; the snowflake means true winter-rated in Canada.
DOT code: week and year the tire was made. Last four digits: WWYY. Old rubber gets hard and cracks even with tread left.
Max pressure: NOT the pressure you should drive on daily. Use the door sticker for normal inflation.
Treadwear, traction, temperature grades: UTQG ratings on some tires; rough comparison tool, not gospel.
Directional arrows: tire must mount spinning one way. Common on performance and winter tires.

Reading size when shopping winter tires

In Ontario a lot of drivers run a dedicated winter set. Same size rules apply. Some people downsize one inch on steel winter wheels narrower, taller sidewall to save money and get a bit more snow flex. That's a deliberate choice; the overall diameter still needs to stay close to factory spec.

When you swap seasons, label which tires go where if they're directional or staggered (different sizes front vs rear). Mixing that up is a bad day.

Common mistakes when reading tire size

Confusing aspect ratio with millimetres. 65 is not 65 mm, it's a percentage of width.
Ignoring load index. Cheaper tire with lower load rating is not a bargain.
Matching only three numbers. 225/65R17 on a minivan is not the same tire as 225/65R17 on a sports sedan if load and speed ratings differ.
Trusting worn tires on the car. Previous owner may have put on whatever was cheap.
Forgetting the spare. Full-size spare should match; donuts and temp spares have their own rules, check the manual.

Quick reference: what each part means

Using 205/55R16 91T as a compact car example:

205 = 205 mm wide
55 = sidewall is 55% of 205 mm tall
R = radial construction
16 = fits 16-inch wheel
91 = load index (~615 kg per tire)
T = speed rating up to 190 km/h

Write down what's on your door sticker and keep it in your phone notes. Next time you're at a tire shop or scrolling Kijiji, you'll know exactly what you're looking at.

Bottom line

Tire size is a sentence, not a random code. Width, profile, wheel size, load, and speed each tell you something important. You don't need to be a tire engineer, just read the string left to right and compare it to your door sticker before you spend money.

Get the size right and everything else brand, tread pattern, winter vs all-season gets easier to decide.