How To Understand Tire Sizes

The tire sizes that are branded on the tire sidewall provide a lot of information about the tire’s dimensions, intended purpose, load capacity, and high-speed / high-temperature durability? That means there are a lot of differences between tires sizes.

Now, the question is how to understand the tire sizes by vehicle?

Every digit represents something. In this article, I am going to write the meaning of those digits/letters so that you can easily understand the difference between the tire sizes. So read the article till the end.

Here is how you can know the tire sizes of your vehicle.

Service Type

Most of the tire sizes start with a letter. This first letter indicates the service type. That means which types of car or vehicles are suitable for these types of tire, or what types of services for they are specially designed. Also, you can find the letter elsewhere in the entire sizes.

How to Read Tire NumbersThe P Size Tire

Some of the tires have the ‘P’ letter in the sizes, for example, P225/50R16 91S. Do you know what does it mean?

The size that starts with ‘P’ means that it is a ‘P’ metric size tire that is specially designed for the passenger vehicle such as minivans, cars, sports utility vehicle and other small pickup trucks that have generally ½ or ¼ load capacity.

These type of ‘P’ metric sizes tire was started using in the late 1970s and now it is the most used tire sizes over the world.

T Size Tire

If you see that a tire number size that starts with ‘T’ like T125/90D16 98M then this means it is a ‘Temporary Space’ size, that is designed for only temporary use until a flat tire is replaced or repaired.

How To Read Tire Numbers

LT Size Tire

You will find some tire size character that starts with ‘LT’ such as LT245/75R16 108/104S. This indicated that it is a ‘Light Truck’ metric size. These types of tires are designed for those vehicles, that have the capacity of carrying weighty cargo or other towing huge trailers such as full-sized vans, sports utility vehicle and other ¾ to 1-ton capacity heavy-duty pickup truck.

These tires are designed for delivering the capacity of the substantial reserve. So that it can tolerate the extra stresses while carrying heavy cargo.

ST Size Tire

This one is another tire size. For example, ST225/75R15. This indicates ‘Special trailer service.’ These kinds of tire sizes are designed to use on the car trailer, boat or other utility vehicle trailers. It should not use on car, light trucks or vans.

Another LT Size Tire

If a tire size ends with ‘LT’ like 7.50R16LT 112/107Q, then you need to understand that these are either “Wide Base”, “Flotation”, or “Numeric” Light Truck size. These tire sizes are designed for those vehicles that have the ability to carry towing trailers or heavy cargo. Such as medium, light to a heavy-duty pickup truck and sports utility that has ½, 1/3, or 1-ton carrying capacity.

This is actually kind of similar to the tire sizes that start with LT.

Difference Between Tires Sizes?C Size Tire

There are some tire numbers that end with ‘C’, such as 95/70R15C 104/102R. This ‘C’ signifies that the tire is a ‘commercial’ tire that is designed for use on the delivery truck or vans. You know those vehicles are capable of carrying the weighty load.

No Letter

You will find some tire sizes that does not start or end with a letter, for your help 225/50R16 92S. These tire numbers are called ‘Metric’ size. Also, few people call it ‘Euro Metric’ size tire. The reason is that these tires are originated in Europe.

These tires are generally used on car European cars, however, these are suitable for the sports utility vehicle and vans as well. The dimensional size of this Euro-metric size tire is similar to ‘P’ metric tire size, but the difference is the load capacity.

Besides those tire sizes, there are few others include:

Section Width

Following the service type letter, the three numeric digits indicate the Section width in mm. For example, if you see a P225/50R16 91S tire size then you will find out that after P letter, there is a three digit which is 225. This means, the width of the inner the widest point of the inner sidewall to the widest point of the outer sidewall is 225 mm. if you converted it onto inches then it will be 25.4 inches.

The Aspect Ratio of Sidewall

In general, following the section width, there are 2 digits that indicate the aspect ratio of the tire.

If we check the previous tire size which was P225/50R16 91S, then we will find out that after the section width, there is 50. This is called the aspect ratio. This is telling us that the sidewall distance, from the rim to the tread outside, is 50% of the section width. The lower aspect ratio means that the sidewall is shorter. The higher aspect ratio means that the sidewall is longer.

Since here our aspect ratio is 50% and the section width is 225 mm or 25.4 inches, we can say that the sidewall height of the tire is 12.7 inches. If the aspect ratio is 25% then we can say that the sidewall height is 6.25 inches.

Internal Construction

By seeing the letter after the aspect ratio, you can know the internal construction of the tire. Most of the time you will see an ‘R’. But a few times, you can find B or D as well.

The R indicates that the tire has Radial construction. These Radial construction tires are most popular all over the world. 98% of the total tire sell today are these Radial construction tires.

If you see that the R is replaced with a D then you can be sure, that tire has the bias ply construction. That means the internal tire body plies crisscross on the diagonal pattern. The light truck, as well as spare truck applications, are this construction tires.

If the R is replaced with B then it is sure that the tire body plies are not only crisscrossed on the diagonal pattern but also there is an additional reinforcing belts layer under tread area. These types of tires are called Belted construction tire and now these kinds of tires are rare.

Speed Rating

Nowadays, the speed rating that is included in tire numbers is the only Z. In most of the cases, you will find this speed rating after the two digit aspect ratio. From 1991, all other speed ratings are mentioned in the service description of the tire.

Understand The Tire Sizes

Service Description

From 1991, every speed rating must have a service description. Basically, the service description mentions the load index of the tire and speed rating of the tire as well.

Tire and Wheel Diameter

If you notice this P225/50R16 91S example carefully, you will find out that after the R, there are 2 numbers. Did a question come to your mind that what those two numbers are?

Basically, it indicates that the tire should be matched to a wheel that has 16 inches in diameter. Basically, the tire comes with 8, 10, 13, 14, 15, 17, 18, 19, 20, 22, 23, 24, 26, and 28. These are the common tire numbers that are mostly used in vans, cars and other vehicles.

However, you might find some exceptional tire sizes. One thing I want to mention that, these exceptional tire should not be combined with any traditional tires and wheels. These are exceptional. These wheels and tires have a half inches express diameters such as 14.5, 15.5, 17.5 and 19.5 inches. Also, you can see some tires and wheels that are expressed in millimeter as well.

Final Verdict

That’s all. I hope you have clearly understood what the differences between tire sizes are. As you can see, a single number can change the whole features and use of a tire. Every singular digit has its own features and characteristic. So those people who do not know the differences, do not know the tire as well. So from now, I hope you will have no problem regarding understand or know properly about the tire sizes and how to read the tire numbers. If that happens, my article is successful.

By the way, recently we have researched and listed the best all-terrain tires which might help you to find your next all terrain tire.

If you are not clear yet, then watch this video to understand better:

Stay tuned to know more about automotive and gears at thegrabbable.com. cheers!

How To Understand Tire Sizes
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Summary
Henry Smith
 

Henry Smith is an SAE certified mechanic engineer and Technical Writer at thegrabbable.com, A resident expert and professional, Smith is passionate regarding all matters about automotive and prefers to share his knowledge. He has good experience in all kind of automotive accessories. He has worked as a Chief Mechanical Engineer in some reputed automotive garage firm.

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