How to Read Your Tyre Size: A Complete Guide

Understanding the numbers and letters on your tyre sidewall is essential for safety, performance, and buying the right tyres. This guide makes it simple.

Why Your Tyre Size Matters

Your vehicle’s tyre size is precisely specified by the manufacturer for good reason. Fitting the wrong size can have serious consequences.

Four Key Reasons

Affects Safety Systems

Incorrect tyre sizes can affect braking distances, handling characteristics, and the proper functioning of safety systems like ABS (Anti-lock Braking System) and ESP (Electronic Stability Program). These systems rely on precise wheel speed data that can be thrown off by incorrect tyre sizes.

Impacts Performance & Economy

Wrong tyre sizes can lead to inaccurate speedometer and odometer readings, meaning you could be driving faster (or slower) than you think. They can also negatively impact fuel economy and overall vehicle performance.

Can Damage Your Vehicle

Tyres that are too large can rub against the wheel arches, suspension components, or brake calipers, causing damage to your vehicle. Tyres that are too small may not provide adequate load capacity.

Legal & Insurance Issues

Fitting incorrect tyre sizes can result in an MOT failure. More seriously, it may invalidate your car insurance in the event of an accident, leaving you personally liable for all costs and damages.

Decoding Your Tyre Sidewall: What Do the Numbers Mean?

Let’s break down a common tyre size: 205/55 R16 91V

205 – Tyre Width

This first number is the width of the tyre in millimeters, measured from sidewall to sidewall when the tyre is properly inflated and mounted. In this example, the tyre is 205 millimeters wide. Common widths range from around 145mm (for small city cars) to 315mm or more (for high-performance sports cars and SUVs).

55 – Aspect Ratio (Profile)

This second number is the height of the tyre’s sidewall, expressed as a percentage of the tyre’s width. In this case, the sidewall height is 55% of 205mm, which equals 112.75mm.

A lower aspect ratio (e.g., 35, 40, 45) means a lower profile tyre with a shorter sidewall. These are often found on sports cars and provide sharper handling but a firmer, less comfortable ride. A higher aspect ratio (e.g., 60, 65, 70) means a taller sidewall, which provides a more comfortable ride and better cushioning over bumps, commonly found on family cars and SUVs.

R – Construction Type

This letter indicates the tyre’s internal construction. ‘R’ stands for Radial, which is the standard construction for virtually all modern passenger car tyres. Radial tyres have internal ply cords that run radially (perpendicular to the direction of travel), providing better grip, fuel efficiency, and ride comfort compared to older bias-ply tyres.

You may occasionally see ‘B’ (Bias-belt) or ‘D’ (Diagonal), but these are extremely rare on modern vehicles.

16 – Rim Diameter

This number is the diameter of the wheel rim in inches that the tyre is designed to fit. This tyre fits a 16-inch wheel. Common rim diameters range from 13 inches (small city cars) to 22 inches or larger (luxury SUVs and performance vehicles).

It’s crucial to match this number exactly—a tyre designed for a 16-inch wheel will not fit a 15-inch or 17-inch wheel.

91 – Load Index

This is a numerical code that indicates the maximum weight each tyre can safely carry when properly inflated. The load index does not represent the actual weight in kilograms; instead, it’s a code that corresponds to a specific weight capacity.

In this example, a load index of 91 means each tyre can carry a maximum of 615 kilograms. For a four-wheeled vehicle, this means a total load capacity of 2,460 kg (615 kg × 4).

It’s essential to fit tyres with a load index that meets or exceeds your vehicle manufacturer’s specification. Fitting tyres with a lower load index than recommended is dangerous and illegal.

3 Easy Ways to Find Your Vehicle’s Correct Tyre Size

Now that you understand what the numbers mean, here’s how to find the right size for your specific vehicle.

Method 1: Check Your Tyre Sidewall

The most direct method is to look at the tyres currently fitted to your car. The size will be clearly written in large numbers on the sidewall of the tyre. You’ll usually find it near the rim, and it will look something like “205/55 R16 91V.”

Walk around your car and check at least two of your tyres to ensure they are all the same size. If you find that your tyres are different sizes, refer to Method 2 to confirm the manufacturer’s recommended size, as someone may have fitted incorrect tyres in the past.

💡 Pro Tip: If your tyres are very dirty or the markings are worn, clean the sidewall with a damp cloth to make the numbers easier to read.

Method 2: Look Inside Your Car

Your vehicle manufacturer provides a tyre information placard or sticker somewhere on your car. This placard shows the original, manufacturer-approved tyre sizes and the correct tyre pressures for your vehicle. This is the most reliable source of information.

The placard is usually located in one of these places:

Method 3: Use Our Online Tyre Finder (Easiest!)

Beyond the Size: Other Markings to Know

Your tyre sidewall contains other important information that can help you make informed decisions.

The DOT code is a series of letters and numbers that includes the manufacturing date of your tyre. Look for a four-digit number at the end of the DOT code (e.g., “3223”). The first two digits represent the week of manufacture, and the last two represent the year. So “3223” means the tyre was made in the 32nd week of 2023. Even if a tyre has plenty of tread depth remaining, the rubber compound degrades over time. We recommend replacing tyres that are over 10 years old, regardless of their condition, as the rubber may have become brittle and unsafe.

M+S and 3PMSF (Winter Capability)

**M+S** stands for “Mud and Snow” and indicates the tyre has some capability in winter conditions. However, this is a basic designation and doesn’t require rigorous testing. **3PMSF** (Three-Peak Mountain Snowflake) is a more stringent symbol that looks like a snowflake inside a mountain. Tyres with this symbol have passed specific performance tests in severe snow conditions and are considered true winter tyres. If you live in an area with harsh winters or plan to drive in snowy conditions, look for tyres with the 3PMSF symbol.

[Learn more in our Winter Tyre Guide →](/guides/winter-tyres/)

Run-Flat Markings

Run-flat tyres are designed to be driven for a limited distance (typically 50 miles) at a reduced speed (usually 50 mph) even after a complete loss of air pressure. This allows you to reach a garage safely without needing to change the tyre on the roadside.

Different manufacturers use different codes to indicate run-flat tyres:
– **RFT** (Run-Flat Tyre) – Bridgestone, Firestone
– **ROF** (Run On Flat) – Goodyear, Dunlop
– **ZP** (Zero Pressure) – Michelin
– **SSR** (Self-Supporting Run-flat) – Continental

If your vehicle came with run-flat tyres, you should replace them with run-flat tyres to maintain the vehicle’s design characteristics.

Now You’re an Expert, Let’s Find Your Tyres

You now have all the information you need to confidently find the perfect tyres for your car. Use our tyre finder tool to see our full range of premium, mid-range, and budget options, complete with prices and EU label ratings. Or give our friendly team a call for expert advice tailored to your specific needs and driving style.

[Browse Our New Tyre Range →](/services/tyres/new-tyres/)

Questions? Call our Mitcham garage on [07904 962265](tel:+447904962265).