How to Read Your Tyre Size: A UK Driver's Guide

Published 2026-05-29 · Park Royal Tyre & Alignment Centre

Your tyre size sits in plain sight on the sidewall, written as a code like 205/55 R16 91V. Each part tells you the width in millimetres, the profile, the construction, the wheel diameter, and how much load and speed the tyre safely handles.

Key takeaways

The sidewall code (e.g. 205/55 R16 91V) is the quickest way to read your exact tyre size.
The first number is section width in millimetres; the second is the aspect ratio as a percentage.
"R" means radial construction, and the two-digit number after it is the rim diameter in inches.
The load index and speed rating at the end show maximum weight and maximum permitted speed.
Always cross-check the sidewall against your handbook or door-pillar placard before buying.

Where do you find the tyre size?

You'll find the size moulded into the outer sidewall of every tyre, raised in the rubber so it's readable even when dirty. Look for a string such as 205/55 R16 91V running around the edge. It sits among other markings like the brand, the pattern name, and the DOT code.

The same size appears on all four tyres if your car runs a single specification. Give the rubber a quick wipe if road grime hides the digits. We've found that a torch helps, because the lettering is the same colour as the tyre and catches shadows rather than light.

What does each number in the tyre size mean?

Each block in the code describes one physical measurement or rating. Using 205/55 R16 as the example, the tyre is 205mm wide, has a sidewall height that is 55% of that width, uses radial construction, and fits a 16-inch wheel. Read left to right, the numbers move from the tread inwards.

Section width (205)

The first three digits give the nominal section width in millimetres, measured across the widest part of the tyre. A "205" tyre is roughly 205mm wide. Wider tyres usually offer more grip but can affect fuel use and steering feel.

Aspect ratio (55)

The two digits after the slash are the aspect ratio, expressed as a percentage. A "55" means the sidewall height equals 55% of the section width. Lower numbers mean a shorter, stiffer sidewall; higher numbers give a taller, more cushioned profile.

Construction and rim diameter (R16)

The "R" confirms radial construction, used on virtually all modern car tyres. The number straight after it, "16", is the wheel rim diameter in inches. This figure must match your wheels exactly, so a 16-inch tyre will not fit a 17-inch rim.

How do you read the load index and speed rating?

The final numbers and letter set the safety limits. In 91V, the "91" is the load index and "V" is the speed rating. Load index 91 corresponds to 615kg per tyre, and a "V" rating permits speeds up to 240km/h (149mph), according to standard ETRTO load-index and speed-symbol tables.

Load index

The load index is a coded number, not a weight in kilograms. You convert it using a standard reference table. Common car values include 91 (615kg) and 95 (690kg). Fitting a tyre with a lower load index than your vehicle requires is unsafe and can be illegal.

Speed rating

The speed rating is a single letter showing the maximum speed the tyre is built to sustain when correctly inflated and loaded. Familiar examples are T (190km/h), H (210km/h), V (240km/h), and W (270km/h). Never fit a rating below your manufacturer's recommendation.

Many drivers assume a higher speed rating is always "better". In practice, matching the rating to your car keeps ride comfort and wet braking balanced, because higher-rated tyres often use stiffer compounds you may not need.

Where else is the tyre size listed besides the tyre?

Your vehicle records the recommended size in several places, which is handy when tyres are worn or mismatched. The most reliable sources are the owner's handbook and a placard fitted to the car, both required to show manufacturer-approved sizes and pressures for your exact model.

Check the driver's door shut, the B-pillar, the inside of the fuel-filler flap, or the glovebox lid for a sticker. When customers arrive unsure of their fitment, the door-pillar placard settles it within seconds, and it also lists the correct cold inflation pressures. The handbook adds any winter or spare-wheel sizes the maker approves.

What if the numbers on my tyres differ?

Mismatched sizes are common and worth investigating. Front and rear tyres can legitimately differ on some performance and rear-wheel-drive cars, a setup called a staggered fitment. However, if your four tyres show random sizes for no documented reason, that points to past replacements with the wrong specification.

Differences in speed rating or load index across an axle can compromise handling and may not be road-legal. UK guidance requires tyres on the same axle to be of compatible structure. If you're unsure whether your fitment is correct, have it checked rather than guessing.

Browse our car tyre range to compare sizes, or read more about fitting options on our tyres overview page.

Frequently asked questions

Can I fit a different tyre size to the one listed?

You should only fit a size approved by your vehicle manufacturer. Changing width, profile, or rim diameter alters the rolling circumference, which can affect the speedometer, gearing, and safety systems. If you want a plus-size or winter option, confirm it appears in your handbook or check with a fitter first.

What does the letter before the rim size mean?

The "R" before the rim diameter stands for radial construction, where the casing plies run radially across the tyre. Almost all modern car tyres are radial. You may rarely see a "B" for bias-belted or a "D" for diagonal on older or specialist tyres, but these are uncommon today.

Is the load index a weight in kilograms?

No, the load index is a coded number that maps to a maximum weight using a standard table. For example, an index of 91 represents 615kg per tyre, while 95 represents 690kg. Always match or exceed the load index your vehicle requires; never fit a lower one.

Where is the easiest place to find my tyre size quickly?

The sidewall is fastest, since the full code is moulded onto every tyre. If the tyres are worn or replaced incorrectly, the driver's door-pillar placard or owner's handbook gives the manufacturer's recommended size. Both also list the correct tyre pressures for your specific vehicle.

Still unsure which figures matter for your car? The team at Park Royal Tyre & Alignment Centre in Park Royal, London NW10 7TR is happy to read your sidewall and confirm the right fitment. Call us on 020 3886 2355, message us on WhatsApp at 07476 586 589, or use our contact page to book in.