Tyre Size Explained: Read the Sidewall Like a Pro

A clear breakdown of every number and letter on your tyre sidewall, plus how to choose a safe replacement size.

Getting tyre size explained properly is one of the most useful things you can learn as a driver, because that short string of numbers on the sidewall controls your grip, your speedometer accuracy, your ride comfort and even whether the tyre is legal for your car. A code like 205/55 R16 91V looks cryptic, but every character has a precise meaning. Once you can read the sidewall markings confidently, you will never overpay a fitter or accept the wrong tyre again.

This guide walks through each part of the code, explains load index and speed rating, covers safe plus sizing, and shows you exactly where to find your correct size.

Where to find your tyre size

You actually have two sources, and they do not always agree:

1. The tyre sidewall itself tells you what is currently fitted. Look for the largest code moulded into the rubber, usually near the rim.
2. The vehicle placard tells you what the manufacturer recommends. Check the sticker inside the driver's door jamb, inside the fuel-filler flap, or in the owner's handbook.

Always treat the placard as the authority. A previous owner may have fitted the wrong size, so the sidewall only tells you what someone chose, not what is correct. If the two differ, investigate before you buy.

Decoding the tyre size code

Take the common example 205/55 R16 91V. Here is what each element means:

Section width (205)

This is the nominal width of the tyre in millimetres across the widest part of the tread. A larger number means a wider footprint, which generally improves dry grip but can slightly increase fuel consumption and road noise.

Aspect ratio (55)

The aspect ratio is the sidewall height expressed as a percentage of the section width. With a 205 width and a 55 profile, the sidewall is roughly 205 x 0.55 = about 113 mm tall. A lower number (say 45 or 40) means a shorter, stiffer sidewall: sharper handling but a firmer ride and more vulnerability to pothole damage.

Construction and rim diameter (R16)

The R stands for radial, the standard construction where the internal plies run radially across the tyre. The 16 is the wheel rim diameter in inches. This number must match your wheel exactly; a 16-inch tyre will not seat on a 17-inch rim.

Load index and speed rating

The two characters at the end are safety-critical and often overlooked.

The load index is a coded figure, not a direct weight. Each number maps to a maximum load per tyre in kilograms. A few common values:

The speed rating letter shows the maximum sustained speed the tyre is designed for:

Never fit a tyre with a lower load index or speed rating than the manufacturer specifies. Doing so can be illegal, may void insurance, and puts the tyre under stress it was not built for. You can go higher on the speed rating safely, but the load index should always meet or exceed the original.

How to read your tyre size step by step

1. Park safely and turn the wheels so the sidewall faces out for an easy read.
2. Find the largest moulded code, for example 205/55 R16 91V.
3. Note the three digits before the slash: that is your width in mm.
4. Note the two digits after the slash: that is your aspect ratio.
5. Confirm the letter and number for construction and rim size.
6. Record the final number and letter: load index and speed rating.
7. Cross-check every value against the door-jamb placard.

Write the full code down before shopping. Buying on width and diameter alone is a common and expensive mistake. For quick reference conversions and pressure checks, our free car tools can help you double-check the numbers.

Plus sizing: changing wheel size safely

"Plus sizing" means fitting a larger-diameter wheel with a lower-profile tyre while keeping the overall rolling diameter almost identical. Keeping that outer diameter constant is the golden rule, because it preserves your speedometer accuracy, gearing feel and, on many cars, correct ABS and stability-control behaviour.

A sound rule of thumb: for every inch you add to the rim, drop the aspect ratio by roughly 10 points and add around 10 to 20 mm of width. For example, moving from 205/55 R16 to 225/45 R17 keeps the rolling diameter within about one percent.

Benefits and trade-offs of going bigger:

• Better dry grip and sharper steering response from a wider, stiffer tyre.
• Firmer, noisier ride and greater risk of kerb and pothole damage.
• Higher cost: low-profile tyres in large sizes typically run $150 to $350 each versus $90 to $160 for common mid-size tyres.

If the rolling diameter changes more than about two percent, your speedometer and odometer will read incorrectly, so verify the maths before committing. If a warning light appears after new wheels, an OBD-II code lookup can help you identify whether a wheel-speed sensor or calibration fault is involved.

Other important sidewall markings

Beyond the size code, the sidewall carries information worth knowing:

• DOT date code: the last four digits show the week and year of manufacture. "3823" means week 38 of 2023. Tyres degrade with age even when unused; many makers advise replacement after six years and against use beyond ten.
• Treadwear, traction and temperature ratings: a comparative durability and grip grade system moulded near the size.
• M+S or the snowflake symbol: indicates mud-and-snow or genuine severe winter capability.
• Direction arrows: on directional tyres, they must roll in the marked direction to clear water correctly.

When to get professional help

Mixing tyre sizes across an axle, running mismatched ratings, or ignoring the load index can affect handling in ways that are hard to feel until an emergency. Because tyres are the only part of your car touching the road, have any size change, unusual wear pattern, or safety-critical fitting checked by a qualified mechanic or tyre specialist. For more on keeping rubber in good shape, see our tyre care and maintenance guide.

Frequently asked questions

Can I fit a different tyre size than the one specified?

You can, but only within careful limits. The safest approach is to keep the manufacturer's size. If you plus size, keep the overall rolling diameter within about two percent, never drop below the specified load index or speed rating, and make sure the wider tyre does not rub the bodywork or suspension at full lock. Anything outside those bounds should be checked by a professional.

Do all four tyres have to be exactly the same size?

All four should share the same width, aspect ratio and rim diameter, and ideally the same brand and model, especially on all-wheel-drive vehicles where mismatched rolling diameters can damage the differential. At a minimum, the two tyres on the same axle must match in size, construction and speed rating.

What happens if my tyre size makes the speedometer wrong?

A larger rolling diameter makes the speedometer read slower than your true speed, while a smaller one reads faster. It also skews your odometer and can confuse ABS and traction systems. Keeping any size change within roughly two percent of the original diameter keeps these errors negligible; larger changes may need a professional recalibration.