How to Decode a Tire Spec: A Complete Guide

Choosing the right tire for your car is a fundamental necessity to ensure better performance and overall safety of a vehicle.

By :
HT automatic office

Oct 11, 2022, 3:44 PM

Knowing a tire is easy by decoding the alphanumeric code written on it. (Reuters)

Although we love driving cars and riding motorcycles, there are many little things that we don’t pay attention to, but these little pieces of information actually play a key role in big things. One of these pieces of information is the tire size description. We often check the tire specification details scribbled on the side of the tires. But only a few people who enjoy driving a car or riding a motorcycle understand exactly what the spec describes.

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Here is a complete guide to decoding the tire size specification.

Let’s take an example of an ideal tire size specification written in the car manual: P225/70/R16 91S. In this case, you need to find the right size and the ideal tire for the car using the code mentioned above. This alphanumeric code contains a total of seven hidden components and describes the car’s ideal tire width, load index, rim diameter, aspect ratio and speed rating.

The first letter of this alphanumeric code, P, signifies that the tire is intended for passenger vehicles. These can be cars, minivans, SUVs or any other type of passenger vehicle.

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

After the slash, the second visible digit in the alphanumeric code is the aspect ratio of the tire, which is provided as a percentage. Tire manufacturers calculate a tire’s aspect ratio by dividing its rim height by the width. If a tire has an aspect ratio of 70, as in this case, that means its height is 70% of its width. The lower the aspect ratio of a tire, it provides vehicle handling performance advantages over higher aspect ratio tires.

After the aspect ratio, there is a letter which indicates the type of internal construction of the tire, which maintains its stability. As in this case, it’s R, which stands for Radial. In some cases, the code mentions D, which indicates a diagonal construction. The radial construction of a tire means that the internal plies of the tire are oriented in a radial direction, from one bead to the other, which is essentially perpendicular to the axis of rotation.

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

The next numerical value is the tire’s load index. This means how much weight in kilograms the tire can hold when fully inflated. Interestingly, the numerical value does not indicate the exact weight that the tire can support. This is why it is described as a load index instead of just a load capacity. The number actually corresponds to a specific load capacity listed in an index. The index starts with 60 and ends with 179; the load index figures represent cart load capacities between 250 kg and 7,750 kg.

The last value is alphabetical in the code mentioned above and signifies the speed rating of the tire. The speed rating letter corresponds to a specific speed capability based on a standard. In this case, the speed rating mentioned as S means that the tire can withstand a speed limit of up to 180 km/h. If the letter is mentioned as R, it can withstand a speed of up to 170 km/h.

How to decode tire specs

Step 1 :

Check the alphanumeric code

2nd step :

The first alphabetic value means the character of the car, whether it is a passenger vehicle or a commercial vehicle.

Step 3:

The next numerical value signifies the width of the tire.

Step 4:

The next numerical value comes to signify the aspect ratio of the tire.

Step 5:

The next alphabetical value indicates whether the tire is radial or diagonal in terms of its construction pattern.

Step 6:

The next numerical value represents the diameter of the rim that supports the tire.

Step 7:

Next to the rim diameter value is the tire load index represented by a numerical value.

Step 8:

The last element of the alphanumeric code is the speed rating indicated by an alphabetic value.

Date of first publication: October 11, 2022, 3:44 PM IST