Reading a Tire Size Codes
Until around 1978, tire sizes were indicated by alpha-numeric codes, such as D78-14. Then, with the adoption of the metric system, the codes changed drastically. Today, the new codes are much longer and provide more information. Tire dealers are required by law to have leaflets available that explain tire grades and ratings. The leaflets also tell you which metric codes replace the old alphanumeric designations.
For example, here’s the information provided by a common "P-metric" tire code, P205/75R-14:
P = Type of vehicle. (In this case, P = Passenger. Other codes include LT for Light Truck and T for Temporary or spare tire.)
205 = Tire section width, measured across the tread, from one sidewall to the other, in millimeters. (In this case, the tire width is 205.)
75 = Aspect ratio or tire series the ratio of the tire’s sidewall height to its width. (In this case, the tire’s sidewall height is 75 percent of its width.) Tires with a low series (less than 70) are referred to as low-profile tires and’ have a short sidewall.
R = Tire type. (R = radial, B = bias-belted, D = diagonal or bias-ply, and E = elliptic. In this case, the tire is a radial.)
14 =Diameter of the wheel, measured in inches. (In this case, the diameter is 14 inches.)
Codes may differ slightly from one tire to another. In addition to the P-metric code, there are also European metric codes, which can be as simple as 155SR13 or as complex as 185/70R14 885, but the data they disclose is pretty much the same.
Sometimes an additional letter appears between the profile and tire type, such as P205/75SR14. The additional letter S refers to a speed rating. The speed rating represents the safest maximum speed for the tire. It tells you nothing about a tire’s construction, handling, or wearability, but measures the tire’s ability to endure the high temperatures that high speeds create. Here’s a list of what the speed rating letters mean:
F = 50 mph ; G = 56 mph ; J = 62 mph ; K= 68 mph ; L= 75 mph ; M = 81mph ; N = 87 mph ; P = 93 mph ; Q = 100 mph ; R = 106 mph ; S = 112 mph ; T = 118 mph ; U = 124 mph ; H = 130 mph ; V = 149 mph ; Z = 149+ mph
you may also find the speed rating listed in conjunction with the tire’s load index, such as 97H. In this case, the 97 is the load index, and the H is the speed rating. If your car originally came fitted with speed-rated tires, you should replace them with speed-rated tires.