Most commonly you will find the letters “P” or “LT” as the first part of the tire size. These indicators show if the tire is intended for use on passenger vehicles or light trucks. In other words, “P” tires meet the standards for use on passenger cars, while “LT” tires provide the durability needed for light trucks.
Next is the 3-digit number indicating the tire footprint’s width in millimeters. This is measured from one sidewall to the other and gives the width of the tire’s road contact area. So, if the number of 265 that means the tire is 265 millimeters wide.
After the width and slash, you will find the tire’s aspect ratio. This number is the ratio of the tire cross-section’s height relative to its width. It is a percentage that shows the height of the sidewall. Therefore, if the tire size is P265/70R17, then the 70 shows that the sidewall’s height is 70% of the width of the tire.
Most passenger car and light truck tires will have “R” after the aspect ratio. This indicates that the tire has a “radial” construction. This is the most common construction of such tires. However, other tire types can also have “bias” construction, in those cases, it is usually represented by a dash or an “x”.
The tire’s rim diameter is the size of the wheel it is intended for shown in inches. While the other sizes can be varied a bit, this number has to match perfectly. A 15-inch tire will not fit on a 17-inch wheel. Therefore, if the tire size is R265/70R17, it is for a 17-inch wheel only.