Tire size numbers are usually written on the sidewall in a format like P215/65R15
The first letter shows the tire type
P means passenger vehicle
LT means light truck
T means temporary spare
The first number is the tire width in millimeters
The slash separates width and aspect ratio
The second number is the aspect ratio
The aspect ratio is the sidewall height as a percentage of the width
The letter after the aspect ratio shows construction type
R means radial
D means diagonal or bias-ply
The last number is the wheel diameter in inches
Example: 215 means 215 mm wide
Example: 65 means the sidewall height is 65% of 215 mm
Example: R means radial construction
Example: 15 means the tire fits a 15-inch wheel
Some tires also include a load index after the size
The load index shows how much weight the tire can support
Some tires also include a speed rating after the load index
The speed rating shows the maximum safe speed category
Example format: 215/65R15 95H
95 is the load index
H is the speed rating
Some tires may include extra markings for load range, season, or special use
Check the vehicle manual or placard for the correct tire size