How to read a tire size
In a size like 225/65R17, 225 is the width in millimetres, 65 is the sidewall height as a percent of that width, R means radial, and 17 is the rim diameter in inches.
Updated
Start with the size on the sidewall and the door placard
A passenger-tire size is printed on the sidewall and, for the size the vehicle was designed around, on the placard on the driver door jamb or in the owner’s manual. Read both. The sidewall tells you what is mounted now. The placard tells you what the vehicle maker specified.
A typical size looks like 225/65R17 102H. Each part is a measurement or a rating. It is not a quality score.
225 / 65 R 17
225 is the section width in millimetres, from sidewall to sidewall, at the tire’s design width. It is not the width of the tread alone.
65 is the aspect ratio. The sidewall height is 65 percent of the width. On a 225 mm tire that is 225 × 0.65, about 146 mm. A lower aspect ratio is a shorter sidewall.
R means a radial tire: the body plies run radially, from bead to bead. You may also see D or a dash for older bias construction. Run-flat construction is usually a separate mark on the sidewall, not the letter R.
17 is the rim diameter in inches. The tire has to match the rim diameter. A 17-inch tire does not mount on a 16-inch wheel.
The load index and speed rating that follow the size
In 102H, 102 is the load index and H is the speed rating. The load index is a code, not the load in kilograms. Use the tire maker’s load table if you need the kilogram figure. Do not guess it from the number.
The speed letter is the maximum speed the tire is rated for at its rated load when it is inflated correctly. It is not a recommended driving speed. Common letters include S, T, H, V, W, and Y. A higher letter is a higher rating, not a better tire for every car.
XL, or extra load, means that size is built for a higher load than the standard-load version. The vehicle placard still decides the pressure and the load you should run.
Changing size is a fitment question
A different width, aspect ratio, or rim diameter changes the overall diameter, the clearance, and sometimes the speedometer. Stay with the placard size unless the vehicle maker lists an alternate. Tire-Finder’s vehicle search looks up sizes for a make and model. It does not invent a stock count.
Common questions
Is the number on the sidewall the pressure I should use?
No. The pressure molded on the sidewall is a maximum for that tire, not the vehicle’s recommended pressure. Use the placard on the driver door jamb, the fuel door, or the owner’s manual.
Does a higher speed rating mean a safer tire?
It means the tire is rated for a higher speed under its rated load. The right rating is the one the vehicle specifies. A higher letter does not tell you how the tire stops on snow or how long it will wear.