how to determine tire size

How To Determine Rv Tire Size ? [ All You Need To Know About ]

Don’t you always think that it would be great if I could save my money by cutting the extra expenses?

Yes, obviously. I do. Everyone does this.

But have you ever think that you don’t know that what type of tire is best for your vehicle. And you have to change the tires after a certain period and the tires don’t long last.

So you have to know how to determine the size and capabilities of a tire. By this, you will be able to maintain ensure the safety and overall performance of your vehicle.

So, let’s find out the system by how you will be able to determine the size of the tire like the width, diameter, and other things also. P345/80R18 81S. Are you getting confused? Maybe you are thinking why this!!!

Believe me, in every tire on the front you will find something like this.

Don’t think too much. Because I am going to explain what is the meaning of this numbers and letters and what is the relation between the tire size and these numbers.

First letter

Let’s talk about the first letter ‘P’. If you find in the tire ‘P’ by engraving. , This means for ‘P-metric’ which is being disclosed by the short form ‘P’. ‘P-metric’ defines that the tire has been designed for passenger type car. So you can use this tire for the main passenger car along with cars, SUV ’s, minivans and sometimes light-duty pickup.

If you find the first letter ‘Lt’ instead of ‘P’ then the meaning will be like for ‘Lt-metric’. The ‘Lt-metric’ means you can use the tire for light trucks. You can also use the tire for heavy cargo and also pulling trailers.

Among these, there can be like ‘T’ or ‘ST’. These bear also some meaning and they are like the temporary and special trailer.

The width of the tire

Then comes the numbers. The first three number of the series remind you about the information of the size of the tire. These numbers define the width and that is in millimeter.

The measurement has been counted from one sidewall to another. So the number and the first letter ‘P345’ bear the meaning that the tire is for passenger type car and has the width of 345 millimeters.

Aspect ratio

After these numbers and letters, you can see the slash and after that a number ‘80’. This is a percentage and the name of this percentage is named by aspect ratio.

Dividing the tire heights by the width you can find the aspect ratio. So the 80% defines that the tire’s height is 80% of its width.


Now a letter comes and that is ‘R’. This ‘R’ defines that the tire is being constructed as radial.

You can also find in the market as diagonal or bias ply type tire. That can be shown by ‘D’.

The radial construction defines that the ply cords inside the tire are sectioned in a radial direction. Actually, if you think the axis of the rotation, you will find the internal ply cords as perpendicular.

The diameter of RIM

The diameter of the rime can be found by the next number. You have to catch this number as the inch.

So, the number ‘18’ bears the meaning that a tire will suit a rim with an 18-inch diameter.

Load – index

The next number will tell you how much weight the tire will be able to bear.  The weight is shown in the pound by this number.

But one thing is important, this number doesn’t carry the precise number and that’s why this is being called index number. The number can be from 1 to 150 and indicate the weight from 99 to 7450lbs.

Speed rating

The last and final letter which also indicate the previous load index number. The number indicates speed sequence. A particular speed capacity of the tire has been introduced by the speed rating letter.

‘S’ published letter indicated up to 112 mph and the ‘R’ define up to 106 mph.

So it is important to use the same speed categorized tire otherwise the tire which is slow will disturb the speed.


So, as you have read the article consciously you can now see the number of any tire and can say the capability and determine the size of the tire.

So, continue your journey and never take the wrong decision of buying the tire without knowing if it suits to your vehicle or not.

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