Tyre Measurement Chart: How to Select the Correct Replacement Tyre Size

Replacement tyres can feel confusing once you read the sidewall. A marking like 205/55 R16 91V looks like code, so what do the numbers and letters actually mean?

This guide breaks down the markings, explains tyre dimensions and the tyre speed rating, and shows how to pick the right tyre size for cars. The goal is to match your vehicle's recommended specifications, not looks or wheel size alone.

Why Understanding Tyre Size Matters

Every vehicle is engineered around specific tyre dimensions. The recommended size reflects its weight, suspension, brakes, handling and wheel design.

Two tyres made for a 16-inch wheel can still differ in width, aspect ratio, load rating and speed rating, so they are not automatically interchangeable.

Check the recommended size in the owner's manual, the tyre-information label or the manufacturer's fitment details before buying.

Tyre Size Explained: What Do the Numbers Mean?

Take a common example:

205/55 R16 91V

Each part carries its own information.

205 – Tyre Width

The first number is the nominal width in millimetres, so 205 means roughly 205 mm. Width affects handling, road contact and rolling resistance, but the right one is always what the manufacturer recommends.

55 – Aspect Ratio

This is the aspect ratio, the sidewall height as a percentage of width. Here the sidewall is about 55% of 205 mm. A lower ratio means a shorter sidewall, and it changes ride comfort and handling, so don't alter it without checking compatibility.

R – Radial Construction

R stands for radial construction, where reinforcing cords run radially across the tyre. Most modern passenger vehicles use it.

16 – Rim Diameter

This tells you the tyre fits a 16-inch rim. People notice it first, yet it is only one part of the full tyre size.

Understanding the Tyre Speed Rating

The final letter, V, is the speed rating. A tyre speed rating shows the maximum speed category the tyre is rated for under specified conditions.

Common speed-rating letters include:

 Confirm any new tyre meets the rating your vehicle manufacturer requires.

What Is the Load Index?

In 205/55 R16 91V, the 91 is the load index, the maximum load the tyre is designed to carry under specified conditions. It has nothing to do with physical size. It must meet the manufacturer's requirements, especially for SUVs, premium vehicles and cars that often carry passengers or luggage. Tyre manufacturers, like Yokohama, often list the load index in the specifications of their tyres.

Tyre Size Chart: A Quick Guide

Here is a simplified tyre size chart for the most common markings: 

This explains the marking system and is not a recommendation for a particular vehicle. Your car may need a completely different size.

How to Find the Correct Tyre Size for Your Car

Start with your vehicle's recommended specifications.

Check the Existing Tyre

Check the sidewall, where the full marking is printed. Don't rely on it alone if the car may once have been fitted with the wrong size.

Check the Owner's Manual

It lists the manufacturer's recommended specifications and is a dependable reference.

Check the Vehicle Information Label

Many vehicles show recommended pressures and, depending on the model, tyre-size details.

Consult a Tyre Professional

An authorised dealer can confirm the right size and fitment.

Can You Change Your Car Tyre Size?

Changing tyre dimensions can alter handling, ride comfort, steering response, ground clearance and speedometer readings, so it is not a simple upgrade. If you want a different size, check that it is approved for your vehicle and keeps suitable load and speed ratings. For most drivers, the manufacturer's size is the safest choice.

What Is the Difference Between Tyre Size and Tyre Dimensions?

People use these terms interchangeably, but tyre dimensions refer to the physical measurements in the full marking. Tyre size combines width, aspect ratio, construction, rim diameter, load index and speed rating. So 205/55 R16 91V says far more than "16-inch tyre." A proper tyre size comparison should weigh the whole specification.

Does Tyre Size Affect Performance?

Yes. Car tyre dimensions affect handling, ride comfort, road noise, braking and rolling resistance, and these should be judged against the vehicle's original engineering. A wider tyre is not automatically better, and a lower-profile tyre is not necessarily an upgrade. The correct tyre meets the vehicle's requirements and suits your driving.

Check the Tyre Speed Rating Before Buying

Never overlook the speed rating when buying replacements. If your vehicle needs a specific rating, the new tyre must meet it, and the same applies to the load index. Read the complete specification, especially for highway or higher-performance use.

Common Mistakes When Choosing Replacement Tyres

Common mistakes include:

• Choosing by rim diameter alone

• Ignoring the load index

• Picking a lower speed rating without checking

• Changing width without checking specifications

• Assuming wider means better

• Buying on price or tread looks alone

When Should You Replace Your Tyres?

Check the tread for heavy wear and the sidewalls for cuts, cracks or bulges. Uneven wear can point to incorrect pressure or alignment problems. If you notice vibrations, steering changes, repeated air loss or visible damage, have a qualified tyre professional inspect the vehicle.

Key Takeaways

Read the full marking, not just the rim diameter, and confirm the recommended size in your manual or vehicle label. Match the tyre speed rating and load index, and avoid changing dimensions casually. When in doubt, ask an authorised dealer, because the right numbers on the sidewall keep every drive safe and predictable.