To read a tire size chart, note the letters at the start of the size line, such as P for passenger vehicle, LT for light truck, ST for special trailer, T for temporary vehicle or C for commercial vehicle. The next numbers represent the width of the tire in millimeters.
The width is measured from sidewall edge to sidewall edge. The width is three numbers long and followed by a backslash. The number following the backslash is the aspect ratio and is the percentage of the width from the inside edge of the wheel to the top of the tread.
The letter following the aspect ratio corresponds to the tire's construction, such as R for radial, B for bias-belt or D for diagonal. This is proportional to the tire's size and is followed by numbers indicating the rim diameter in inches.
Next is the load index number, which is a measurement of how much weight the tire can support. To know what each number means, reference a Load-Carrying Capacity Per Tire chart. For example, if the load index number is 95, the maximum weight the tire can support is 1,521 pounds.
The final letter in the size line indicates the tire's speed rating. This rating measures the speed the tire is meant to sustain for an extended period. The letters S, T, U, H and V correspond to speeds under 150 miles per hour. The letters Z, W and Y indicate speeds over 150 miles per hour.