How to decode a tyre specification: A comprehensive guide

While we love to drive cars and ride motorcycles, there are a lot of small things that we don't pay attention to, but those small pieces of information actually play a key role in major things. One such information is the tyre size description. We often check the tyre specification details scribbled on the side of the tyres. But, only a few people who love to drive a car or ride a motorcycle understand exactly what the specification describes.

By: HT Auto Desk
| Updated on: 11 Oct 2022, 15:44 PM
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Learning about a tyre is easy by decoding the alphanumeric code written on it. (REUTERS)
Learning about a tyre is easy by decoding the alphanumeric code written on it. (REUTERS)
Learning about a tyre is easy by decoding the alphanumeric code written on it. (REUTERS)
Learning about a tyre is easy by decoding the alphanumeric code written on it.

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Here is a comprehensive guide to decoding the tyre size specification.

Take an example of an ideal tyre size specification written in the car manual is P225/70/R16 91S. In that case, you must find the right size and ideal tyre for the car by using the abovementioned code. This alpha-numeric code has a total of seven components hidden in it and describes the ideal width, load index, rim diameter, aspect ratio, and speed rating of the car's tyre.

The first letter of this alpha-numeric code, P, signifies that the tyre is meant for passenger vehicles. These could be cars, minivans, SUVs or any other type of passenger vehicle.

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The first numeric value, 225, is the width of the tyre. Mentioned in millimetres, the value describes the width of the tyre. The width means the number refers to the measurement from one sidewall to another. Thus, a tyre with the measurement P225 is for a passenger vehicle and has a nominal width of 225 mm.

After the slash sign, the second number visible in the alpha-numeric code is the tyre's aspect ratio, which is delivered in percentage. Tyre manufacturers calculate the aspect ratio of a tyre by dividing the height of its rim by the width. If a tyre has an aspect ratio of 70, like in this case, that means its height is 70 per cent of its width. The lower the aspect ratio of a tyre is, it offers vehicle handling performance advantages over higher aspect ratio tyres.

After the aspect ratio, there is a letter that indicates the type of internal construction of the tyre, which maintains its stability. Like in this case, it is R, which means Radial. In some cases, the code mentions D, which indicates Diagonal construction. Radial construction of a tyre means that the tyre's internal ply cords are oriented in a radial direction, from one bead to the other, which is essentially perpendicular to the rotational axis.

The next number is the diameter code, which in this case, is 16. The metric used in this is inches. This indicates the diameter of the rim on which the tyre can be mounted. If we check the example given here, it means the P225/70/R16 91S tyre would fit a rim with a 16-inch diameter.

The next numeric value is the load index of the tyre. This signifies how much weight in kilograms the tyre can support once it is fully inflated. Interestingly, the numeric value doesn't tell the exact weight the tyre can support. This is why it is described as a load index instead of a simple load capacity. The number actually corresponds to a specific load capacity listed in an index. The index starts with 60 and ends with 179; numbers in the load index represent carting load capacities ranging between 250 kg and 7,750 kg.

The last value is an alphabetic one in the abovementioned code and signifies the tyre's sped rating. The speed rating letter corresponds to a specific speed capability based on a standard. In this case, the speed rating mentioned as S means the tyre can support a speed limit of up to 180 kmph. If the letter is mentioned as R, it can support up to 170 kmph speed.

How to decode tyre specification
Step 1 :

Check the alphanumeric code

Step 2 :

The first alphabetic value signifies the car's character, whether it is a passenger vehicle or a commercial vehicle.

Step 3 :

The next numeric value signifies the tyre's width.

Step 4 :

The next numeric value comes signifying the aspect ratio of the tyre.

Step 5 :

The next alphabetic value shows if the tyre is radial or diagonal in terms of its construction pattern.

Step 6 :

The next numeric value portrays the diameter of the rim that holds the tyre.

Step 7 :

Next to the rim diameter value is the tyre load index portrayed through a numeric value.

Step 8 :

The last component of the alpha-numeric code is the speed rating shown through an alphabetic value.

First Published Date: 11 Oct 2022, 15:44 PM IST
TAGS: car
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