How to read car tyre sizes correctly? Frequently asked questions2 December 2018
May it be because you got a flat tyre or just because you need to change for winter tyres, when changing your car tyres, you’ll need to know their size. Luckily, it’s always written on tyres, so you just have to repeat it to your provider. Still, it is always useful to know exactly what it means, as some measures must compulsorily be respected while other can slightly vary. Here are some explanations to frequently asked questions on tyre specifications.
What do flat tyre sizes look like?
When reading a flat tyre size to your provider in order to buy a new one, you’ll have to tell him various letters and numbers for him to know exactly what tyre type you need. This should look like something like this :
This is always written on your tyres, so you just have to copy and repeat it. Still, it can be useful to know what these figures mean, as some must absolutely be respected while other can vary a bit.
What do these measurements correspond to?
Tyre class – P, LT, T, ST, C or nothing
The first letter corresponds to the tyre’s class. ‘P’ stands for ‘passenger’ car tyre, ‘LT’ means ‘light truck’, ‘T’ corresponds to ‘temporary’ (spare wheel tyres), ‘ST’ stands for ‘special trailer’ and finally ‘C’ means ‘commercial’ vehicle tyre. Note that in Europe, you won’t find this letter. No letter indicates indeed that it’s a European metric tyre.
Section Width – 155 to 335
The first 3-figures number indicates the width of the tyre. It can go from 155mm to 335mm. This measurement can slightly vary. You can choose it according to the width of the rim. Here is a table that gives you the minimum and maximum section width according to your rim size.
|Rim size||Minimum section width||Optimal section width||Maximum section width|
|5,0 inches||155 mm||165 to 175 mm||185 mm|
|5,5 inches||165 mm||175 to 185 mm||195 mm|
|6,0 inches||175 mm||185 to 195 mm||205 mm|
|6,5 inches||185 mm||195 to 205 mm||215 mm|
|7,0 inches||195 mm||205 to 215 mm||225 mm|
|7,5 inches||205 mm||215 to 225 mm||235 mm|
|8,0 inches||215 mm||225 to 235 mm||245 mm|
|8,5 inches||225 mm||235 to 245 mm||255 mm|
|9,0 inches||235 mm||245 to 255 mm||265 mm|
|9,5 inches||245 mm||255 to 265 mm||275 mm|
|10,0 inches||255 mm||265 to 275 mm||285 mm|
|10,5 inches||265 mm||275 to 285 mm||295 mm|
|11,0 inches||275 mm||285 to 295 mm||305 mm|
|11,5 inches||285 mm||295 to 305 mm||315 mm|
|12,0 inches||295 mm||305 to 315 mm||325 mm|
|12,5 inches||305 mm||315 to 325 mm||335 mm|
Aspect Ratio – 25 to 80
Next 2-figures number indicates the aspect ratio, meaning the height of the tyre sidewall in % of the tyre section width. In our example, ’60’ means that the tyre sidewall measures 60% of the tyre section width.
Tyre construction – R, D, B
Next letter determines the tyre construction type. Nowadays, it is most often an ‘R’ for ‘radial tyre’, but it used to be as well ‘D’ for ‘diagonal’ (less and less used today) or else ‘B’ for ‘belted’.
Wheel diameter – 10 to 21
The next two-figures number tells us about the wheel (or rim) diameter, which is also the tyre inside diameter. Today’s standard wheels can go from 10 to 21 inches. The measure must absolutely not be changed.
Load index – 60 to 125
The final two-figures number shows the load index, which determines the maximum weight the tyre can bear. It can go from 60 to 125, which are not actually the total weight the tyre can stand, but an indicator. For instance, a load index of 60 corresponds to a maximum charge of 250kg. This measure can vary, provided that you select a higher load index than the initial one.
Speed rating – almost any letter
Final letter tells us about the speed rating, which determines the maximum speed the tyre can stand, in km/h. It can be any letter, sometimes followed by a number, from A1, which corresponds to 5km/h, to Y, which can stand up to 300km/h. Same as the load index, you can choose another speed rating as long as it’s higher than the original one. Note that this rule does not apply to winter tyres.
You may find a letter just next to the wheel diameter, indicating another load capacity. ‘C’ indicates a 6-ply load capacity, ‘D’ is an 8-ply and ‘E’ is a 10-ply rating. Most often, you don’t have this letter, which means the tyre has a standard 4-ply rating.
You may also find a ‘tubeless’ mention, meaning that the tyre functions without a tube.
Can I change only one tyre?
When you change for winter tyres, you’ll obviously change both, even your four tyres. But when you have a flat tyre, you may think that you can change only the damage tyre. This is wrong! You must never change only one tyre, as it will affect your car symmetry. This can lead to damaging your car’s steering and balance. Note that it is reprehensible by forces of order in case of police check.
Do both tyres fit both side?
No. When changing your tyres, each tyres fit only one side. There is a very simple way to know to what side corresponds a tyre: designs on car tyres always convert to one direction, which must fit with your car’s front. If you invert tyres side, it will result in an advanced erosion of your tyres.
How do I recognize winter tyres?
That’s an easy question, and the answer is so simple you’ll wonder how you didn’t come up with it by yourself: winter tyres often have snowflakes designed on it. It’s just as simple as that. Additionally, you can check for the letters ‘M+S’, standing for ‘mud and snow’. They are always indicated on winter tyres.
Can I drive with a flat tyre?
Theoretically, yes, you can drive on a flat tyre. Your car will still start up and all mechanics will run as usual. However, it is a very bad idea, as your car won’t be symmetric and balanced any more. This will result in many factors that will make your car very hard to drive, like your car not going straight any more, pulling down to the flat tyre side. This can be very dangerous, especially in case of bad weather conditions where you could lose control of your vehicle and cause damage to not only you, but other drivers too. Also, as when you change only one tyre, driving with a flat tyre will result in damaging your car’s steering and balance prematurely. However, remember to never stop to change a tyre if you can’t find a safe place for it. You can still drive a short distance in order to reach such a place.