Decoding the Sidewall Lettering on Your Tyres
Posted by: Dan on 2/12/2015
Buying tyres can, at times, be a challenging task. If you’re a car enthusiast - or good friends with your local tyre supplier - you may find it easy to go and collect a new set each time. If you regard yourself as more of a driver than a mechanic, however, it may be a more challenging assignment. This is especially the case when it comes to the sidewall lettering on your tyres.
Yet, as opposed to thinking ‘ignorance is bliss’ or feeling it’s not all that important to maximise your auto knowledge, learning how to read tyre lettering can help you better understand your driving needs, as well as make your next tyre purchase more straightforward.
When you look at a tyre, you’ll first see a name or identifier - such as Goodyear, Michelin, Firestone. While it's useful to be able to spot the name or a tyre logo if you have a favourite brand - and also a corresponding series of model names which may be unique to the brand - moving beyond this to the letters alone is where the true information of a tyre’s features are detailed.
Take the following example set of numbers: 225/50/R/92/Y
As we go through this article, we’ll show you the easy way to decode what this lettering means, as well as how you can use this knowledge to your advantage.
To begin your decoding, look for the first number (225 in the above example). This details the section width of your tyre. Usually, the standard number seen across most Australian tyres is 225mm, with variants in 10mm increments either side from 185 to 335 for vehicles of a wider wheel base.
What’s important to understand about this component of your car’s tyre is not the section width itself, so much as the impact it has upon your tyre’s performance. With a wider section width comes a greater surface area and capacity for traction. Conversely, broader tyres diminish your capacity for turning quickly. Any readers that are drag racing fans will immediately understand why the tyres you see on the track are often in excess of 250mm, as they allow for a more rapid straightline speed (and the ability to bring the car to a quicker stop upon braking).
The next number after the section width is the aspect ratio, which spells out the relative size of your tyre in height to width. Though needing to calculate precise fractions in tyres is generally the responsibility of the manufacturer’s design teams - and is not needed on your next shop for a new set of wheels - it’s useful to retain an awareness of the aspect ratio when considering what tyres to buy from an aesthetic point of view.
Further, as many car makers use the aspect ratio calculation to determine what tyres provide the most ideal match to a car’s overall presentation‚ knowing what ratio was envisaged as ideal is particularly important for vintage and classic car enthusiasts. Oftentimes, a simple Google search will give you a number of brands that make tyres suited to a vintage vehicles original design. But what if you have a particularly rare or obscure car? In that case, giving a tyre maker or fitter the original aspect ratio will help them find the best modern match for your car restoration.
Following the aspect ratio, you should then see a single letter - and look for the letter R, which stands for “radial.” While radial tyres were not always the universal standard in tyre construction, essentially all new standard road tyres within Australia are constructed as radials.
Using a three ply form of construction - so that the tyre’s cord plies are criss-crossed over each other in a triangular fashion - the radial tyre offers the best combination of safety and usability. Accordingly, while some off road 4WD users and racers retain a genuine use for cross-ply tyres, they’re no longer considered the safest fit for general driving use.
Therefore, though there are many things that make for a safe driving experience, make it a priority to look for the R on every tyre that you buy.
Once you’ve moved beyond the radial indicator lettering, you should then see numbering that indicates the tyre’s load index. For Australians who use their daily vehicle in a variety of ways, this is a notable part of the tyre wall lettering to pay particular attention to.
A passenger car will typically have a load index of a number between 90 and 105. Yet, it’s important to note that these numbers do not specify the weight your car can carry. Instead, they indicate a corresponding number within the load index, with 90 signifying that your tyres can carry 600kgs, and an increase in 15kgs follows on in each number thereafter (so 91 equals 615kgs, 92 equals 630kgs and so on).
Generally speaking, if you’re just seeking to drive around the city and attend to regular duties with your vehicle, the occasional extra weight on your tyres outside the ideal on the load index won’t really impact their performance. If you anticipate changes in the weight in which you’ll carry in your car on your next set of tyres, however, then discussing with your supplier the ideal load index for you car is prudent.
Finally, the last letting of your tyre’s sidewall details information concerning its speed classification. Similar to the load index, each ascending letter that appears on the tyre corresponds with an incremental increase in speed symbols. In turn, while the speed symbol is detailed separately to the load index, it is interrelated, as it indicates the top speed at which you car can safely travel using the tyres in accordance with the relative weight it is carrying.
Starting with the letter J and a top speed of 100kms, the classifications continue with K (110kms), L (120kms) and so forth, topping out at ZR and a top speed in excess of 240kms. It’s worthwhile to keep in mind when looking at speed symbols on a tyre that there’s some variance in their use across different States and territories in Australia, in accordance with local laws and speed limits. Therefore, if you’ve recently moved or are buying tyres out of your home state, it’s best to just check before purchase that the tyre type you’re purchasing is a similar symbol to the one you’ve used prior.
With recent statistics from England showing 11,000 car accidents occurred over just one winter due to tyre-related causes, ensuring your wheels are safe and properly set up is essential. Understanding your tyre’s lettering also means that you stand to benefit from your greater knowledge surrounding the four wheels that move your vehicle along the road. Use this information to keep you safe by ensuring your auto is always equipped with the appropriate tyres.