Awhile back, we talked about how there are just so many different tyre sizes on the market. Now with so many different sizes available, choosing your next tyre can be confusing or even intimidating at times. So today, we’ll explain what all those numbers really mean and how to choose the right tyre for your car.
For today, we will be focusing mainly on the different size specifications and how they affect your tyre selection. We will alsotouch on a few other common markings found on tyres so that you have a better idea of what they mean.
So on your typical road tyre, you will see a string of letters and numbers like this:
These are the main ones that most people will need to look at when choosing a tyre. The first number is the nominal section width, the second is the aspect ratio and the third number is the rim diameter. So for a tyre to even fit your car properly, you will have to get these 3 numbers correct and here’s how:
Nominal section width
So to be more exact, the nominal section width of the tyre is the distance of the outermost point on each side of the tyre measured in millimetres. This is different from the tread width that only measures the width of the surface which will be in contact with the road. Tread width will vary between manufacturers as they each use different methods of construction for different types of tyres.
Now this width tends to mostly correspond to the width of the rims you are using followed by the amount of space you have in your wheel well. Especially if you are going to reuse the same rims, you will want to follow as closely as possible to your existing tyre width to get a proper fit. As rims are also available in different widths, changing to a wider or narrower rim will allow you to use wider or narrower tyres as long as your wheel well has enough space for it to turn fully and unobstructed.
Take note that rim widths do have recommended tyre widths which I will link a reference to it here. Now there are quite a few examples on the internet where people have used undersized tyres for their rims to create a very stretched out look. I would not recommend that at all as it seriously compromises the ability of the tyre to hold on to the edge of the rim and maintain an airtight seal.
The second number here is the aspect ratio and this basically refers to the height of the tyre’s sidewall. This number is a percentage and not an absolute measurement and is based on the nominal section width. So for example, a tyre width of 195mm with an aspect ratio of 55 means that the height of the sidewall is 55% of 195mm which works out to about 107mm.
So since the sidewall height determines the overall diameter of the tyre, it is important to choose the most appropriate aspect ratio when changing to a wider or narrower tyre. As the overall diameter affects not only the clearance of the tyre in the wheel well, but also the accuracy of the speedometer and the handling of the car, the idea is to get the overall diameter as close to the original as possible. Here’s a link to a handy calculator tool that you can use to easily compare different tyre sizes and see what works best for you.
Now the third number here is the rim diameter. This one is pretty straightforward, just get the size that corresponds to the rim size you will be using. If you aren’t sure what size yours is, just follow the rim size stated on your existing tyre.
Now a little tidbit here, sometimes you will see an “R” in front of the rim size. It is a common misconception that this “R” stands for “Rim”. What it actually stands for is “Radial” which is the type of construction used for that tyre. These days pretty much all tyres are radial so you don’t really have to worry about this letter. If you happen to be looking for a non-radial tyre for whatever reason, you probably already know enough to not need this guide.
Now we come to some of the less commonly looked at markings and one that most of us tend to overlook but really shouldn’t is the load index. The load index is the number that typically follows the three number size specification we talked about earlier. It will be shown as a 2 or 3 digit number accompanied by a letter. This letter at the end is the speed symbol which we will get into a bit later on.
So why is the load index important? Well this is because it is the maximum load that a single tyre is rated to carry when inflated to 240kpa or 2.4bar. The number itself is not the actual weight and you’ll have to refer to a load index table to determine the maximum load which I will also link right here. So for example a load index of 85 has a maximum load rating of 515kg when inflated to 2.4bar. This amount will decrease if a lower tyre pressure is used hence why it is important to keep your tyres properly inflated at all times.
Now I know that most of us do not even consider the load rating of our tyres and tend to leave it to our tyre shops, which is totally fair, especially when purchasing a reputable brand. Just bear in mind that if you do happen to be choosing one yourself from a more obscure brand, do make sure that the load index matches or exceeds that recommended by your car’s manufacturer or you may be running a risk of tyre failure which is not fun at all.
So you may have guessed that the speed symbol indicates the maximum speed that a tyre can safely run at. This is corresponding to the load index it is rated for. Like before, you will have to look this up from a table which will be linked over here. As an example, a tyre with a speed symbol of “W” has a maximum sustained speed of 270km/h when under load.
Just like the load index, the speed symbol also tends to be overlooked quite often but do still bear in mind to not go below your car’s recommended speed rating if selecting a tyre on your own.
Treadwear, Traction and Temperature Grade
Now the last one we will look at is the treadwear, traction and temperature rating. These are tested under controlled conditions by the manufacturers themselves so your mileage may vary in real world usage. These ratings are better used as a comparison between different tyre models rather than an indication of actual numbers.
So treadwear is basically a measure of how long a tyre can last where a higher number indicates a longer lasting tyre. These numbers are linear in nature so a treadwear rating of 200 should in theory last two times as long as a treadwear rating of 100.
The traction rating is based on straight line braking performance and denoted by a letter grade with AA being the highest and C being the lowest.
Lastly, the temperature rating is about the tyre’s ability to handle heat dissipation and like the traction rating, is also denoted by letter grades from A to C with A being the best and C being the minimum required by law.
How to select a tyre
So when selecting a tyre, you need to consider 2 main things: size and application.
Where size is concerned I would recommend to stay as close as possible to the original tyres that came together with your car. So for the section width, I would try not to go more than 1 size higher or lower than the original tyres as going any larger will be entirely experimental on whether it is able to fit and still retain a full range of motion while the suspension is being compressed.
Also remember to match your rim width to the tyre width properly to ensure a secure fit.
Lastly, stay as close to the overall diameter of the original tyre as possible by using a tyre size calculator to compare.
In terms of application, you will want to determine if you require a more eco friendly tyre that lasts longer and is more fuel efficient or a higher performance tyre that can handle higher speeds and faster braking at the expense of wearing out noticeably faster.
So that about wraps up all the basic things you need to know when selecting your next tyre. If you are ever unsure about anything, do consult a professional or your preferred tyre shop in order to get the safest and most optimal recommendations for your car and your needs.