Car Tyre Sizes Explained: What the Numbers and Letters Really Mean

Choosing the correct tyres for your vehicle is about much more than finding the right fit. Your tyres play a vital role in your car's safety, handling, fuel efficiency and overall performance. Using the wrong tyre size or specification could even affect your insurance cover or cause your vehicle to fail its MOT.

Whether you're replacing worn tyres or simply want to understand the markings on your tyre sidewall, this guide explains everything you need to know.

Why Choosing the Correct Tyres Matters

Every vehicle is designed to use a specific tyre size and specification. Manufacturers carefully match tyres to a vehicle's suspension, braking system, steering and electronic safety features to deliver the best possible performance.

Using tyres that don't meet the manufacturer's recommendations can lead to:

• Reduced handling and braking performance

• Incorrect speedometer readings

• Increased tyre wear

• Poor fuel economy

• Potential MOT failures

• Insurance complications if incorrect tyres contribute to an accident

Your recommended tyre size can usually be found in your vehicle handbook, inside the driver's door frame or on the fuel filler flap.

Understanding Tyre Markings

A tyre sidewall contains all the information you need about its size and performance. For example:

205/55 R16 91V

Each part of this code has a specific meaning.

Tyre Width

The first number (205) is the tyre width measured in millimetres from one sidewall to the other.

A wider tyre generally provides:

• Increased grip

• Improved cornering stability

• Better traction in dry conditions

However, wider tyres may also increase road noise and rolling resistance.

Aspect Ratio (Tyre Profile)

The second number (55) represents the tyre's profile or aspect ratio.

This figure shows the height of the sidewall as a percentage of the tyre's width. In this example, the sidewall height is 55% of 205mm.

Lower-profile tyres typically offer:

• Sharper steering response

• Improved handling

• A sportier appearance

The trade-off is that they usually provide a firmer ride because there is less sidewall to absorb bumps in the road.

Radial Construction

The letter R indicates radial construction, which is fitted to the vast majority of modern vehicles.

Radial tyres are designed with reinforcing cords positioned at 90 degrees to the direction of travel. This construction allows the tread and sidewall to work independently, helping to deliver:

• Better ride comfort

• Improved grip

• Greater durability

• Enhanced fuel efficiency

Wheel (Rim) Diameter

The number following the construction type (16) is the wheel diameter measured in inches.

This must match the diameter of your alloy wheel or steel wheel exactly. A tyre designed for a 16-inch wheel cannot be safely fitted to a 17-inch wheel.

Load Index

The load index (91 in our example) shows the maximum weight each tyre is designed to carry safely.

Every vehicle has a minimum load rating specified by the manufacturer. Choosing tyres with a lower load index than recommended can reduce safety and increase the risk of tyre failure, especially when carrying passengers or luggage.

You can fit tyres with a higher load rating, but never one that is lower than the manufacturer's specification.

Speed Rating

The final letter (V) represents the tyre's speed rating.

This indicates the maximum speed the tyre has been tested to carry its maximum load safely under controlled conditions.

Some common speed ratings include:

Although UK speed limits are well below these figures, it's still important to choose tyres with the correct speed rating for your vehicle.

Why Tyre Speed Ratings Matter

Tyres generate heat as they roll and flex against the road surface. At higher speeds, this heat builds more quickly, placing greater demands on the tyre's construction.

A correctly rated tyre is designed to cope with these conditions while maintaining grip, stability and braking performance.

Driving on tyres with a lower speed rating than recommended may:

• Reduce handling performance

• Increase tyre wear

• Raise the risk of tyre failure

• Affect your vehicle's insurance cover

For these reasons, it's always best to replace your tyres with the same speed rating specified by the manufacturer or a higher approved rating where appropriate.

Can You Fit Tyres with a Higher Speed Rating?

Yes. Many drivers choose tyres with a higher speed rating than originally specified.

Higher-rated tyres often provide:

• Improved steering precision

• Better cornering performance

• Enhanced braking response

• Increased stability at motorway speeds

While they can offer performance benefits, they are not necessarily "better" for every driver. The best tyre is one that matches your vehicle, your driving style and the conditions you regularly encounter.

Will Incorrect Tyres Affect Your MOT?

Tyres form an important part of every MOT inspection.

During the test, your vehicle's tyres are checked for:

• Correct size where applicable

• Adequate tread depth

• Damage or excessive wear

• Overall condition

• Tyre Pressure Monitoring System (TPMS) operation on vehicles equipped with the system

Tyres that are damaged, excessively worn or unsuitable for your vehicle could result in an MOT failure.

Need Help Choosing the Right Tyres?

If you're unsure which tyres are suitable for your vehicle, our experienced team is here to help. We'll recommend tyres that match your manufacturer's specifications and your driving needs, whether you're looking for premium, mid-range or budget options.

At Sandicliffe, we're committed to helping you keep your vehicle safe, reliable and performing at its best.