How to read your car tyre size

May 10, 2023

Think all car tyres are the same size? You couldn’t be further from the truth! Car tyres are manufactured in a wide range of sizes and it’s important to use the right size of tyre on your car. 

If you are looking to update your tyres and need to know which size to purchase, it’s relatively easy to find out the ideal size for your car—provided you know where to look. It’s also easy enough to check the size of the tyres on your vehicle. 

In this article we’ll run through how to read your car tyre size, how to work out what is the best tyre size for your car, and why it all matters.

Your car tyre size

In most cases, your car’s wheels should have been fitted with the correct size tyre, especially if it has its original tyres, or has had tyres fitted by a tyre fitting specialist. But if you are the new owner of a vehicle and you’re not sure of the history of your tyres, it’s worth checking to see if your tyres are the recommended size for your wheels.

Reading the tyre

The size of a tyre is printed on your tyre’s sidewall. A group of numbers and letters printed on the sidewall indicates your tyre’s size, speed rating and load capacity.

Before we look at an example of a tyre sidewall, remember you can always check the website of each tyre brand to see how they present their tyre sizes. There is a standard format, but sometimes you see something different. If you are unsure of exactly how to read the markings on your tyre’s sidewall, you can do a web search of the exact markings you see, and you should find an explanation.

Example tyre markings : 205 / 70R 15C

In this example, we see a series of numbers and letters. Let’s break it up.

205 refers to the tyre width (from sidewall to sidewall) in millimetres (mm).

70 refers to the aspect ratio, also known as the tyre profile. This is the ratio of the tyre’s cross-section to its width. In this case, the aspect ratio is 70, which means it is 70% of its width.

R this refers to ‘radial’ construction.

15 refers to the wheel diameter, in inches.

A tyre sidewall may also include extra information about its load index and speed rating. For example, on this Dunlop tyre 205 / 55  R16  91V, the 91V means it has a load index of 91 (it can carry 615kg), and a V speed rating (V means it can travel at a maximum speed of 240 km/h). 

For more on load indexes and speed ratings, see below.

Tyre Load Index

Every tyre will differ in regards to how much weight it can carry when properly inflated. That’s where the tyre load index comes into play. The index tells us the maximum weight a car tyre can carry. The higher the tyre load index number, the more weight it can support.

It’s really important to ensure new tyres you buy are capable of carrying the same weight as the tyres your vehicle originally came with. Choosing tyres with less load carrying capacity may lead to problems.

Note, if tyres are over or under-inflated, the load carrying capacity will not be accurate.

As mentioned above, the tyre load index is printed on the tyre, after the tyre’s size information. To see how the index rating translates to the load carrying capacity, you’ll need to look at a Tyre Load Index Chart (there’s one available on this Wikipedia page).

Speed Ratings

The speed rating of a tyre is the maximum speed at which the tyre can travel when carrying its maximum load. It’s represented by a single letter, or an A with one numeral (A1-A8). 

Refer to a Speed Rating chart to see the speed that corresponds with the speed rating number you see on your tyre.

Why having the right tyre size matters

There are no accidents when it comes to designing a vehicle and its components, so of course tyre size is planned. Using the right size tyre helps ensure your vehicle is safe on the road. 

How does tyre size affect your vehicle? 

Overall, having bigger wheels increases your vehicle’s traction and stability, as more tyre surface area is connected to the road. However it does mean they will rotate fewer times on the road than a smaller tyre would, which will slow down your acceleration slightly, and affect your odometer and speedometer readings, rendering them inaccurate. Larger tyres may also reduce your car’s mileage, as they are heavier.

Fitting tyres that are too small can also affect odometer and speedometer readings, amongst other potential mechanical problems. In addition, a smaller tyre will rotate more times than a larger tyre, resulting in premature wear and tear on the tyre itself.

Therefore, it’s recommended to use the tyre size recommended by the vehicle manufacturer. In addition, you should ensure that all tyres on your vehicle match in terms of size and tread. Mis-matched tyres can affect the control of your car, and can also damage your vehicle’s suspension.

Recommended tyre size

Your car’s manufacturer will recommend a tyre size for your vehicle. This size will appear in your vehicle owner’s manual. You can also find out the recommended tyre size on the tyre placard inside your vehicle’s door jamb on the driver side. If you can’t find these, check your manufacturer’s website.

If you still aren’t sure, or you just would rather someone else handle it, then a specialist from a mobile tyre shop can expertly match tyres to your vehicle. 

To get someone to do a tyre size reading and order the right tyres for you, contact Jim’s Mobile Tyres. Not only is Jim’s a tyre repair service that comes to your door, but we are a mobile tyre shop that can order tyres from quality suppliers for a range of vehicle types. We also handle tyre fitting expertly.

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