Understanding Tire Size
You can determine your vehicle's proper tire size by looking in the Owner's Manual. Unless the size of the actual wheels (rims) on the vehicle have been changed, this is the size recommended by the manufacturer. The tire size can also be found on the sidewall of your current tires. This obviously assumes that the manufacturer's recommendation has been followed during any previous tire replacements, so the Owner's Manual should always be your definitive source for this information.
P would indicate a passenger car tire. Other options would be no P indicting metric sizing (essentially the same as P-sizing, which has its heritage in Europe) or LT for light truck. LT tires are designed for heavier loads and more rugged service conditions.
This is the width across the tire surface in millimeters.
The height of the sidewall section in relation to the width of the tire.
If this number was 50, the tire section is half as tall as it is wide. Short sidewalls deliver crisp handling, tall sidewalls a smoother ride. A smaller number equals shorter sidewall.
R indicates radial construction. Unless you specify the other option, bias (which would have a D (diagonal) or B (belted bias)) in this position instead of an R, you are purchasing a radial tire.
In simple terms this indicates the distance across the "doughnut hole" of the tire. You must match wheel diameter and the tire opening. A tire with a 15" size designation will not work on a 14" wheel, or vice versa. Improper matching of wheel and tire size renders your vehicle extremely unsafe.
Includes the load index (a number) and a speed symbol (a letter). The number specifies weight carrying capability. The higher the number, the greater the load carrying capacity. Never choose a tire with a lower load carrying capacity than the original equipment tire. The speed symbol means the tire has been speed rated on an indoor test wheel:
|N = 87 mph
P = 93 mph
Q = 99 mph
S = 112 mph
|T = 118 mph
U = 124 mph
H = 130 mph
V = 149 mph
|Z = 150+ mph
W = 169 mph
Y = 188 mph