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How to read tyre sizes - what the numbers and letters mean

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Updated 18 Jun 2020

AutoGuru

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The markings on the side of a tyre can resemble hieroglyphics if you aren’t sure what you’re looking for.

Organising a new set of tyres should be high on any pre-road trip agenda.

If you’re about to start searching for a new set of tyres for your car, you need to know the basics, so you can be sure you get the right rubber.

Here’s an easy guide to impressing your mechanic and understanding the numbers, letters and dots on your tyres.

The numbers

The three largest numbers on your tyre’s sidewall are the most helpful.

They indicate the tyre width, profile (height) and the diameter of the wheel they are fitted to.

Let’s get real world on it. Grab a pen and paper or snap a photo of the numbers on your tyre sidewall.

The example we will use here is 235/45R17.

235/45R17

The first number is the easiest and measures the width of the tyre in metric millimetres. 235 means your tyre is 235mm wide.

235/45R17

The second number refers to the profile of the tyre or the height of the tyre’s sidewall.

The number isn’t in millimetres but is instead an aspect ratio measurement of how the height relates to total width.

In our example, the tyres profile is 45% of the total 235mm width.

Unnecessarily tricky? Maybe, but stick with us.

235/45R17

R means radial which refers to a common tyre construction for passenger vehicles.

The final number is a measurement in inches of the diameter of the wheel the tyre will be fitted to.

In this case, 17 means the tyre is fitted to a 17-inch wheel.

Yes, inches. Tyres mix metric and imperial measurements. Just to keep you on your toes.

What's with the dots?

Seeing spots? Don’t worry, the yellow dots on your tyres are there for a reason.

These splotches of primary colour are called ‘balance dots’.

When a tyre is manufactured, perfect balance is extremely rare.

This is why when new tyres are fitted, they are ‘balanced’ with small sticky weights to ensure they roll smoothly and don’t cause vibration or uneven wear.

The yellow dot on the tyre indicates to the tyre fitter where the lightest part of the tyre’s sidewall is.

They will generally line that part of the tyre up with the valve stem, which evens things out and reduces the need for balance weights.

The other numbers

“That’s not all the numbers on my tyre!” I hear you shout.

Take it easy! If you have already mastered the basics, there is definitely more information your tyre manufacturer has offered the advanced tyre diviner.

These include construction date, load rating, treadwear rating and speed rating.

Speed and load rating

Your tyres maximum speed and how much weight is combined on the sidewall.

Typically, this designation will come after the basic size information, and is expressed as a two-digit number (load) and a single letter (speed).

To refer to our example above, a tyre might read 235/45R17 91W.

So, looking at a tyre size of 235/45R17 91W, we now know it can handle 615 kg per tyre, and should not go any faster than 270km/h.

That’s a pretty sporty tyre! What is your tyre’s speed and load rating?

UTQG? What did you just call me?

UTQG stands ‘Uniform Tyre Quality Grading’.

It’s principally an American and European rating from controlled testing, but it can be used to give you an idea of how fast your tyre will wear.

Get down next to that sidewall, you can generally spot these numbers above the tyre size. They comprise treadwear, traction and temperature ratings.

The higher the treadwear rating the longer the tyre will take to wear down. The ‘control’ tyre for this testing is assigned a 100, so a treadwear rating of 300 means your rubber will last three times as long.

These aren’t definitive figures, so tread lightly. Traction is simpler. It is rated from highest traction to lowest as AA, A, B and C. The better the rating, the shorter the braking distance on a wet surface.

Driving makes your tyres hot and the faster/longer you go the hotter they get. Overheated tyres can puncture, or separate.

The temperature is a rating of heat resistance from A, B and C, A being the best. It’s important to remember that these ratings haven’t been created in Australia, and that each tyre is built for a balance of these attributes.

Longer wear can mean lower traction, for and vice versa, for example.

Construction date

Ever noticed a really old set of tyres that just never seemed to wear? That’s not a good thing.

As tyres age, the tread compound hardens. Your tread compound is built to be soft enough to provide you with grip, but not so soft that it wears too fast.

You don’t have a pit crew with a fresh set of Pirellis waiting for you every few laps. As such, tyre makers mark a date on the side of their product, so fitters and drivers know whether or not they are going to get the grip they need.

As a (very) general rule, five years on the shelf means those tyres should be knifed. Manufacturers all use different codes, so you may need to ask for help from the supplier on this one.

The code will normally follow the DOT marking on the tyre. DOT indicates the US Department of Transport has approved the tyre for sale, so if you ask and see the salesman reading from somewhere else, eyebrows should be raised!

Tyre magic

So, you are now an intermediate tyre-wizard. Go forth into the world, spread your knowledge and check out the rest of our tyre content for your official certification* as an advanced tyre warlock. (*certification not official).

Remember, being an intermediate tyre-wizard does not mean you automatically now have the skills to replace your own tyres. We recommend using AutoGuru to book in with a local Tyre Specialist (and certified full-blown tyre-wizard) to replace your tyres. 

AutoGuru can help you find a local tyre expert to address all your tyre needs, you're just a couple of clicks away! With some great tyre deals, it really is the smartest way to get your car tyres replaced. 

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