FAQ : How To Read Tire Size

FAQ : How To Read Tire Size

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It’s a question we receive often and a fair one for those customers who are not used to looking at their tire for measurements, or even if you ever wondered just what the numbers meant. Today we’ll demystify all these numbers and acronyms and make tire shopping a much easier process moving forward.

Information regarding your tires, your tire size, load rating and much more can be found on the sidewall of your passenger tire. Branded or stamped into the rubber, the sidewall provides a lot of great information about your tire’s intended purpose and load capacity as well.

Q: I need new tires, what information do I need?

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A: Looking at your sidewall, you will see a number structured to dictate the tire size. These numbers are what you need to shop for, and purchase your new tires. 225/45/17 is a great example, you will have something similar to this pattern on your tire as well.

Q: But what do those numbers mean?

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A: These numbers signify your tire and wheel diameter, or specifically what size tire you have on your car and what size you will need to purchase in order to replace said tire.

Tire Diameter

Let’s begin with the last number, in our example is the 15 which measures the diameter of the tire in inches. In this case, we are using a 15 inch tire, and will need exactly that size to be mounted on our tire as a replacement. These kinds of tires are called ‘inch rim’ sized tires, but these can also include the ‘half’ inch tires as well.

Tires found on light trucks or medium duty vehicles can often times be tagged with a half inch or decimal point in their diameter size. Sizes such as 16.5LT and 17.5LT can be found on heavy duty trucks, trailers and vans.

Very few tires can also show their diameter in a millimeter sizing, for example a 205/65R390 are called ‘millimetric’ tire sized.

Always confirm the size of tire before mounting and balancing, failure to do so may result in injury or damage to the tire, your rim or yourself.

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Tire Width

The three-digit number usually following the letter at the beginning will signify the width or cross section of the tire in millimeters. In our example, the 205 in the 205/65/R15 gives us the measurements across the widest point of the outer sidewall to the widest point of the inner sidewall when mounted.

Another name for this measurement is also section width, and this value can be converted by dividing the value in millimeters by 25.4 which is the number of millimeters in an inch.

Tire Height

Immediately following the three digit tire width on the tire, should be a double digit number which represents the tire height, otherwise known as the tire series, profile or aspect ratio.

 

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The value of 65 shows us that this tire’s sidewall size is 65% of the tire width and is a measurement of the tire’s section height, or tire profile. We know that the section width on this tire is 205, and 65 percent of the 205 represents the height of the tire. To realize this value, convert the 205mm to inches and then multiply by 65% to determine your tire height.

Q: What do the letters P and LT mean on my tire?

A: These letters signify the service type, or what the tire was originally designed for. In other words, the letters found before the three-digit numeric portion of the tire size tell you what the tire is supposed to do. For example, a “P” found before the three-digit 

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P = Signifies the tire as a p-metric tire, primarily for use on passenger cars and very light duty cars, minivans and SUV’s with a load range of 1/4 to 1/2 ton capacity.

T = Signifies the tire as a temporary spare and should not be used for long periods of time, spare tires should be used until you can reach your local tire facility for replacement / repair.

LT at the beginning of the rating = Signifies the tire as a “light truck” tire designed for usage on vehicles that are capable of carrying heavy loads or towing big trailers. Medium to heavy duty trucks, vans and cargo vans usually utilize this tire and this kind of light truck tire is a smaller cousin of the 18 wheel trailer tires found on large commercial vehicles.

LT at the end of the rating = This means the tires is a light truck size designed for use on vehicles with heavy cargo or towing trailers only. These are known as ‘wide base’ or ‘flotation’ light truck sizes, which allow the vehicle to drive on top of loose dirt or in sandy areas.

C = Signifies a Euro-metric sized tire meant for commercial use only, for vans and delivery trucks capable of extremely heavy loads.

ST = This value represents the ‘Special Trailer’ tire and should not be used on passenger vehicles or trucks / vans. These tires are for use on select towing trailers, boats or utility trailers only.

Internal Construction

Letters that follow the aspect ration, usually ‘R’ can designate how the tire is manufactured and what exactly it’s made out of. In this scenario, the R means that the tire is a radial tire which is the most popular kind of tire in the market today.

Radial construction means that the tire’s body radiates outward from an imaginary center of the wheel, radial tires are the most common of tire, but the internal construction value can also be a D or a B.

D ratings signify a crisscross or diagonal bias ply construction and are usually found in light truck applications or heavy duty trucks or vans.

B ratings signify a belted tire body, meaning that each plied layer is reinforced with belts under the tread area. 

Service Descriptions

The values often found at the very end of your tire measurements, such as 91S or 85W represent the tire’s service description. This is a value that gives the tire a setting in the Load Index and Speed Rating category. For more on this value and speed rating, click here for more information.

Speed Rating

Speed rating has become something of the past, as most of the tires today do not carry the modern day value known as the speed rating. Most tires are only rated Z for Z-Speed rated tires, while often times the internal construction value follows, for example a ZR rating will signify the tire’s speed rating (Z) and the internal construction (R).

For more information on speed rating and internal construction, click here for our How To Buy Tires Guide.

 

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