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TIRE TIPS // BASICS

How to Read Your Tire Size

THE SHORT ANSWER

Your tire size reads like 245/65R17 111T: width in millimetres, sidewall height as a percentage of that width, R for radial, wheel diameter in inches, then load index and speed rating. The size you need is on the placard inside the driver's door, not necessarily on the tire that is fitted.

BY THE RIM2RIM CREW PUBLISHED SEPTEMBER 3, 2026 4 MIN READ

The main string, decoded

Take a common one: 245/65R17 111T.

245 is the section width in millimetres, measured across the widest part of the tire.

65 is the aspect ratio: the sidewall height as a percentage of the width. Here the sidewall is 65 percent of 245 mm, about 159 mm tall. A lower number means a shorter, stiffer sidewall.

R means radial construction. Nearly everything on the road is radial.

17 is the wheel diameter in inches, and it is the one number that has to match your wheel exactly.

111 is the load index, a code for how much weight one tire can carry. 111 is 1,090 kg per tire. This is the number people ignore and should not, especially on trucks.

T is the speed rating, the speed the tire is certified for. T is 190 km/h, H is 210, V is 240. Winter tires often carry a lower rating than the summer tires they replace, which is normal and legal.

Some sizes start with a letter. P means P-metric, a passenger tire. LT means light truck, built with heavier construction for load and gravel. On a three-quarter-ton or anything that tows, LT is usually the right family, and it is not interchangeable with a P tire of the same numbers.

The placard is the authority, not the tire on the car

Open the driver's door and look at the jamb. The placard there lists the sizes and the cold inflation pressures the vehicle was built around. That is the reference.

The tire currently fitted may not match it. Vehicles get sold, sizes get changed, and the last shop may have fitted whatever was in the rack. If the sidewall and the placard disagree, start from the placard and have someone look at why.

What you can change, and what you cannot

Wheel diameter has to match the wheel. Everything else has a little room, as long as the overall diameter stays close, usually within about three percent, and the load index and speed rating meet or beat the placard.

Sizing down for winter is the common change here, and it is a good one. A narrower tire on a slightly taller sidewall cuts through slush and snow instead of floating on top of it, and it rides better on frost heaves. We work the alternate size out from the placard rather than guessing.

Going wider or lower for looks costs you sidewall, which costs you comfort and gives you a wheel that finds every pothole on Milligan Drive in April. It also changes your speedometer if the overall diameter moves too far.

The other markings that matter

M+S is a tread pattern claim. It is not a snow test and does not make a tire a winter tire.

The three-peak mountain snowflake is a test result: at least a 110 traction index against a reference tire on medium-packed snow. That is the marking that matters in October, and both winter and all-weather tires carry it.

Treadwear, traction and temperature are the UTQG codes. Treadwear is a relative wear number from the manufacturer's own testing, useful within one brand and misleading across brands. Traction A or AA is wet braking. Temperature is heat resistance.

Max load and max pressure are the tire's own limits, not your target pressure. Your target is on the door placard.

The DOT code ends in four digits: week and year of manufacture. 3225 means the 32nd week of 2025. Tires age whether or not they are driven, and rubber that has sat for years hardens the same way cold rubber does. Six years is a fair time to start watching, ten is the outside limit for most manufacturers, and a bargain set built five years ago is not a bargain.

Reading a truck sidewall

Trucks add two things. Load range letters, C, D and E, describe the construction and the pressure the tire is built to carry at. And LT sizes sometimes appear in the older flotation format instead, as in 285/70R17 or 33x12.50R17, which is 33 inches tall and 12.5 inches wide.

If you tow, the load index is the number to protect. Dropping to a lighter tire because it was in stock is how people end up with a sidewall failure on Highway 22 with a trailer behind them. Our truck and LT tire page covers the ranges we fit most.

Quick answers

Where is my tire size? On the sidewall, and on the placard inside the driver's door. Use the placard as the reference.

Do all four tires have to be the same size? On almost every vehicle, yes. Some performance cars use a staggered setup, wider at the back, and that is by design.

Can I fit a different size than the placard? Yes, within limits, if the overall diameter stays close and load and speed ratings meet the original. Ask before you buy.

What does the DOT date tell me? The week and year the tire was built. Watch tires past six years, and replace them by about ten regardless of tread.

What if my tires do not match the placard? Find out why before you buy. Someone may have fitted the wrong size, or the vehicle may have had a wheel change.

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