Can you easily decipher the string of numbers and letters stamped on the side of a tire to find the tire size? Or does it look like a whole different language to you? When you’re trying to find the right size of tires to fit your vehicle, you may find yourself wondering where to find the size on a tire, how tire sizes are measured, and if tire sizes even matter. To find out how tire sizes work, read on for the Tire Research blog’s Tire Size Breakdown.

The vehicle you drive was designed to use a specific tire size, or range of tire sizes, for optimal performance.

Where to Find the Size on a Tire

First things first – to read the tire size, you have to know where to find it. Tire sizes are stamped on the sidewall of the tire, and will look something like the image to the right. These numbers and letters will give you a wealth of information about the tire, and help you know if a tire will fit your car. 

To find which tire size fits on your vehicle, check the inside frame of the driver’s side door, close to where you’d find the car’s VIN number, and make sure to check the size of the tires that are currently on your vehicle, in the event that your vehicle has been modified to be able to use a different tire size.

Tire Size Number and Letter Meanings

Vehicle Type
Example: LT
325/65R18 127/124S 10P.R.

The first letter or letters of the tire size indicate the vehicle type that the tire is built for. For passenger vehicles (as opposed to commercial vehicles) this letter will be either P or LT for Passenger or Light Truck respectively. Occasionally, you may see a T, which indicates a Temporary tire, or no letter at all, meaning that the tire originated in Europe. 

Tire Section Width
LT325/65R18 127/124S 10P.R.

The first number represents the tire’s width in millimeters. The width of the tire is measured from the widest point of the inside sidewall to the widest point of the outside sidewall when the tire is mounted on a vehicle. To find what this is in inches, just divide the width in millimeters by 25.4. (For example: 325 mm ÷ 25.4 = 12.8 inches). 

Aspect Ratio
LT325/65R18 127/124S 10P.R.

The second number in the series represents the aspect ratio, or how tall the tire’s profile is, and it is written as the percentage of the section width. This percentage is found by dividing the tire’s sidewall height by its width. In this example, this tire’s aspect ratio is 65% of the tire’s section width. The aspect ratio will affect how much of the tread touches the driving surface, as well as the tire’s steering responsiveness. In general, the shorter the tire (the lower the percentage of the tire’s section width), the more responsive the tire will be to steering.


Internal Construction
LT325/65R18 127/124S 10P.R.

The letter after the Aspect Ratio stands for the internal construction of the tire. The most common construction is R, which stands for Radial construction. Radial tires have cords within the tire’s construction that prevent friction and heat build-up, as well as steel belts, which increase the tire’s stability. The plies on a radial tire are laid across the tire from the inner bead to the outer bead, as if they were “radiating” from the inside rim of the tire. However, you may also see D for Diagonal Bias construction, or B for Belted Bias construction, which uses layers of cord laid diagonally in a criss-cross pattern. This creates tires that are very durable and can withstand a lot, however, they do not give as much driving stability, and experience more friction heat build-up.

Wheel Size/Wheel Diameter
: LT325/65R18 127/124S 10P.R.

The number following the letter representing the tire’s internal construction tells you the rim or wheel size that this tire will fit onto, in inches. In this example, this tire would fit onto a rim or wheel with an 18 inch diameter. 

Load Index and Speed Rating
LT325/65R18 127/124S 10P.R.

The tire’s Load Index and Speed Rating come after the wheel size, and are represented by a two- or three-digit number followed by a letter. The number shows the tire’s load index in pounds, which is the maximum weight a tire can support once it’s inflated. For LT (Light Truck) tires, there may be two numbers, since light trucks often have dual rear wheels. The larger number of the two represents the carrying capacity per tire for single rear wheels, and the lower number represents the carrying capacity per tire if there are dual rear wheels.

The speed rating is a letter that indicates the maximum speed at which the tire was constructed to be driven, according to the manufacturer of the tire. The letter indicates a speed in miles per hour, and the ratings go in alphabetical order, starting with the letter “L” as the lowest (slowest) speed rating, and Z as the highest (fastest). In this example, this tire’s speed rating is S, which indicates that the highest speed this tire should be driven is 112 mph. 

Ply Rating/ Load Range
LT325/65R18 127/124S 10P.R.

The last value listed on the tire size is the tire’s Ply Rating, also commonly called Load Range. The ply rating will be shown as a number from 4 to 12, or the Load Range will be shown as a letter, from B to F. Both of these mean the same thing, and indicate the maximum weight in pounds that the tire can safely carry when the tire is inflated to its maximum pressure. A Ply Rating of 4 is the lowest, and the highest is 12, and a Load Range of B is the lowest, with F being the highest. The higher the Ply Rating or Load Index, the heavier load the tires should be able to carry.

Do Tire Sizes Even Matter?

Yes! The vehicle you drive was designed to use a specific tire size, or range of tire sizes, for optimal performance. Although many driving enthusiasts may use different tire sizes than their vehicles were designed to use for performance or aesthetic reasons, using the wrong tire size for your car puts excess stress on the car’s suspension system, putting essential parts such as shocks and struts at risk of damage. The wrong size tire can also come into contact with vehicle components that it normally shouldn’t, resulting in damage or puncture to the tire itself. Using the wrong size of tire for your vehicle can lead to your tires prematurely wearing out, and often will void your warranty. For these reasons, it’s always important to check the manufacturer’s tire size recommendations for the car you’re buying new tires for. 

Now that you know where to look to find the tire’s size markings, and how to decode their meaning, you’re well equipped to find the perfect replacement tire for your vehicle!

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