How to Read Tire Sizes

Decipher what those numbers on tires mean.

Have you ever wondered what all those cryptic numbers and letters on the sidewall of a tire mean? They are called tire markings and they provide all kinds of useful information, from load capacity, model and construction type, all the way to tire size. You can decipher the tire size markings using this sequence as an example: M + S P 185/80 R15. It is the newest and now widely used metric measuring system. You might want jot down the number sequence from your own car tires and read along or make your own notes so you can go out and ask informed questions when buying tires for your own vehicle.

Things You'll Need

• Notepad and pencil

Instructions

1. Look at the first two letters: "M + S." This mean this tire is designed to properly ride through mud and snow on the city without much difficulty. Be careful, though--this does not mean this is an all-terrain tire unless expressly specified.

2. Compare the next letter to the one on your own tires: "P." It is the one you will see most often in cars around the streets and roads. That is because "P" stands for passenger. Tires with a "P" are specifically made for passenger vehicles.

3. Notice the next three-digit number: 185. It might differ from the one on your tires. This number means that the tire is 185 mm at its section width--that is, from the outside of the tire wall all the way across to the outside of the opposite wall.

4. Refer to the next number after the forward slash: 80. This number is given as an aspect ratio of height to width. Height is measure from the inner-tire circumference to the top of the tread, the part of the tire surface that comes in contact with the road. In the example, the tire's height is 80 percent of the width. The smaller the aspect ratio number is, the wider and shorter the tire will be.

5. Pay particular attention to the next letter. The example comes with an "R" designation. That means it is a radial tire--it has plies running across with extra belts between the plies and the top surface for a stronger tread. Other common designations are "B," for belted bias tires, which provides a smooth ride and traction, and "D" for bias ply tires, which provide a smooth ride on rough terrain but offer weak plies and tread.

6. Take a look at the last number and compare it with yours: 15. This will give the rim size on your vehicle. The example uses a 15 inch diameter rim size. This is the number a salesperson wants when asking for the tire size.

Tips & Warnings

• When buying new tires, take a close look at advertising brochures detailing tire construction. Different tire manufactures use different number of plies and materials to build their tires. Compare designs and models.

• Mainly, car manufactures are using three types of tires on newer vehicles: bias ply tire-D, belted bias tire-B and radial tire-R.

Resources

• Read this Article in Spanish