The tire size is a combination of numbers and letters printed on the sidewall of your tires. Here is an example of how to read it:
P : Tire type. The letter P at the beginning of the tire size means the tire is a P-Metric tire, a reference to tires intended for passenger vehicles made to certain standards in America. The absence of letters at the beginning of the tire size indicates a Euro metric tire. P-Metric and Euro-Metric tires are both suitable for American vehicles tires but may have different load capacities. The letters LT at the beginning or end of the tire size designates a heavy-duty tire designed for light trucks.
245: Tire width. Measured in millimeters from sidewall to sidewall
40: Aspect ratio. Measured as a ratio of the height of the tire's cross-section to its width. The higher the ratio the bigger is the sidewall. In this example of P245/40ZR19, the sidewall of the tire is 40% of 245 mm.
R: Tire construction. R stands for radial, meaning the layer's cords run at right angles to the circumference of the tire (radially). This it the most common type of automotive tire.
19: Wheel diameter. Size, in inches, of the wheel on which the tire can be mounted.
98: Load index. Indicates the maximum load carrying capacity of the tire. When installing new tires, it is important that the load index meets or exceeds your vehicle manufacturer’s specification. The number is associated with a weight in an universal chart.
Y: Speed rating. Designates the maximum speed at which a tire can be driven on. The letter is associated with a top speed in an universal chart