You check your tire size when you might want to buy snow tires or might you need to replace your tires. you find the tire size number are like P225/50R16 91S, What these numbers and letters mean ?
P: “P-metric” size
the tire is a “P-metric” size that was designed to be fitted on vehicles that are primarily used as passenger vehicles. This includes cars, minivans, sport utility vehicles and light duty pickup trucks (typically 1/4- and 1/2-ton load capacity). The use of P-metric sizes began in the late 1970s and they are the most frequently used type of tire size today.
If there isn’t a letter preceding the three-digit numeric portion of a tire size, it signifies the tire is a “Metric” size (also called “Euro-metric” because these sizes originated in Europe). While Metric tire sizes are primarily used on European cars.
The 225 indicates this tire is 225 millimeters across from the widest point of its outer sidewall to the widest point of its inner sidewall when mounted and measured on a specified width wheel.
50: Sidewall Aspect Ratio
The 50 indicates that this tire size’s sidewall height (from rim to tread) is 50% of its section width.
tire has a Radial construction in which the tire’s body plies “radiate” out from the imaginary center of the wheel
16: Tire and Wheel Diameter
The 16 indicates the tire and wheel diameter designed to be matched together.
It is critical that the tire and wheel diameters are always confirmed to match before the tire is mounted on the wheel.
91S: Service Description
represents the tire’s Service Description. A Service Description identifies the tire’s Load Index and Speed Rating.
More detail info please read here
UPdate: Found a nice site to read more info: How to Read a Sidewall