Learn the code: A guide to understanding car tyres

Buying or replacing tyres for your car can be a difficult thing, especially if you do not know what you are looking for. However, the information you need is printed right on the sidewall, in numbers, letters, and words that need a bit of prior knowledge to decipher.  Here is a crash course to the plethora of data stamped on your tyre as well as which ones you should consider when looking for a new set.

The big figures 

On all tyres, there are three sets of numbers and letters. They usually look something like 215/55/R17. These represent the tyre's diameter, aspect ratio, construction type and wheel size. When shopping for new tyres, these are the key things you need to know.  

In this case, the number '215' means the tyre is 215 millimetres at its widest point when it is mounted on the specified wheel size. Wider tyres provide better grip but thinners tyres are more fuel-efficient. As a general rule of thumb, it is safe to fit a tyre up to 20 millimetres wider than stock on the original rim. However, we still recommend consulting a professional before making such changes. 

Some tyres will also have 'P' in front of this number, indicating the tyre is made specifically for passenger cars. Light Truck/SUV tyres will be marked 'LT' while 'ST' means 'Special Trailers' and those tyres are made for large container lorries.  

The '55' after the slash indicates the tyre profile, meaning the sidewall height is 55% of its width. Tyres with a larger aspect ratio ride better but they negatively affect fuel economy and handling. On the other hand, thinner walls look good but you are likely to irreparably damage your rim if you hit a pothole even at moderate speed. 

Then there is the R17 where the 'R' means the tyre is of radial construction. Generally, two other designations can be found, B and D, representing bias-ply and belted respectively. The former is an older method of tyre making and is best avoided. The latter is a bias-ply with internal 'belt' reinforcement, which is better than bias-ply but still inferior to radial in most aspects. Modern radial tyres now generally also come with reinforcement belts, (the number and type of belts should be written plainly on the sidewall) making them overall the best option. 

Finally, the '17' stipulates the rim size - as in the tyre is designed to be put on a 17-inch rim. 

The other big figure 

Right after the 215/55/R17, there should be another alphanumeric code. It should look something like '94H'. 

The numbers and the letter refer to a chart called 'tyre service description,' representing the tyre's load index and speed rating. Following the said chart, we can tell this tyre has a maximum load rating of 670 kilograms and has a speed rating of 210 km/h or 130 MPH. 

Thankfully, many makers now also put this information on the tyre wall in plain text. But in case yours did not, follow the included chart to find out what your tyre's load and speed rating is. 

The red dot

This marks the heaviest spot of the tyre. Some tyre makers do this, many do not. It has little or no relation to the tyre-and-wheel balance.  

Treadwear, traction and temperature 

Treadwear, traction and temperature are three pieces of information that you will find together, usually printed right after or above the load limiter and speed rating. The first one indicates the tyre's treadwear, which is a rough estimation of how long it is likely to last. Much like the load index and speed rating, treadwear is supposed to have its own charts. But in practice, manufacturers do not conform to the same testing standards, leading to different manufacturers using different ratings. What you need to keep in mind is the larger the number, the longer the tyre will last.  

Traction rating denotes how much grip a tyre generates when dragged across wet pavement without rotation. There are four grades, AA, A, B, and C. AA is the best, A is good and also what you will find on most tyres. C is the lowest a tyre can achieve before being declared dangerous. Traction rating is less of an issue if your car has anti-lock brakes as they keep the wheel spinning for better grip even when the emergency brake is pulled. 

Temperature designates how well a tyre dissipates heat.  As laws of physics dictate, the faster something moves, the quicker it gets hot. The rising air pressing inside a hot tire can cause it to violently explode, a catastrophic event that can be fatal for a car at high speed. Temperature ratings are separated into three grades: A, B and C. A grade tyres can withstand speeds over 185 km/h, B can withstand speeds over 185 to160 km/h, while C manages around 160 to 136 km/h. 

Finding the expiry date

There are a disturbing number of people who are not aware of this: Tyres come with an expiration date. The said date is usually six years after the tire is manufactured as after that the tyre slowly begins to fall apart. 

Companies tend to put the manufacture date in different parts, the most common method is to look for the line of code starting with 'dot'. After finding the line, check the last four-digit. The first two digits stand for the week the tyre was made while the other two represent the year. So if a  tyre is marked 'AW30121' it means it was put together in the first week of 2021. Ignore the 'AW3' or any other combination ahead of the four digits as it is an internal company code for the tyre factory itself. 

Aside from shelf life, tyres also wear out with use. And while tyre makers put in 'wear bars' in the tread grooves, they are hard to measure and there is an easier way.

Take a two taka coin (or something 19 millimetres in diameter) and put it inside the tread of your tyres. If the groove is deep enough to touch the book symbol on the coin, you are good. But, if the groove only reaches halfway toward the words 'two 2 taka' (written in Bangla) stamped on the bottom of the book symbol, change the tyre as soon as possible. Depending on your driving habit, do this check each month or every other month to see if your tyres need replacing or not. 

Miscellaneous

Aside from all the general information, some tyres also come with extra marking that symbolises additional capability or specialisation.

For example, the 'mud and snow' or 'M+S' marked tyres have a little extra space between their tread blocks, giving them some limited capability in such terrain. A three-peak mountain with a snowflake indicates a winter (snow) tyre, which while absolutely useless in our country, does come in handy in places where the temperature drops below freezing point during winter. 

2021 Mitsubishi Xpander