Red69Sportsroof

Looking at the sidewall of a tire, you'll see a series of raised numbers and letters such as P 205/55 R 16 88V. Following is a summary of the information you can gain from those markings.


Type of vehicle. The letter at the beginning of the series (figure A) designates the type of vehicle for which the tire is intended to be used. The P in our example indicates that the tire is intended for a passenger vehicle. Other designations include "LT" for light truck and "T" for a temporary or spare tire.

Tire width. The first number in the series denotes the width of the tire, measured across the tread from sidewall to sidewall (figure B). The measurement is expressed in millimeters. In our example, the tire is 205 millimeters wide. The appropriate tire width relates to the width of the rim on which it will be mounted.

Aspect ratio. Following the slash mark, the second number in the series (figure C) denotes the ratio of the tire's sidewall measurement to its height. In our example the 55 means that this tire's height is 55 percent of its width. Tires with lower numbers are referred to as low-profile tires.

Tire type or construction. The second letter in the series is known as the construction code and indicates the type of tire. More specifically, it indicates the type of ply construction in the tire's casing. In our example R indicates that this is a radial tire. Most tires today are steel-belted radial tires, meaning that the cords that make up the belt are made from steel. It also means that the cords making up the belt run radially around the tire -- at an angle perpendicular to the tire's bead. (The bead is the steel hoop made into the inner perimeter of the tire that holds the tire securely on the wheel-rim.) Other possible designations include "B" for belted-bias tires, "D" for diagonal or bias-ply tires or "E" for elliptical. These latter designations are far less common in contemporary tires.

Wheel diameter. The next number is the diameter of the wheel's rim, measured in inches. In our example 16 means that the tire is made to fit on a wheel that is 16 inches wide.

Load index. Following the wheel diameter is a number that corresponds to a maximum load capacity (figure D) -- the amount of weight the tire can safely carry assuming that each tire is filled to maximum inflation capacity. Finding the weight associated with each number requires looking up the number in a load-index chart. In our example, a tire with a load index of 88 has a maximum load-carrying capacity of 1,235 pounds. Most passenger-vehicle tires fall within a load-index range between 75 and 105. That index-range corresponds to load capacities ranging from 853 to 2,039 pounds. The load index is often given in association with a speed rating.

Speed rating. Following the load index, some tires list a speed rating (figure E). This designation generally appears only on performance tires and indicates the maximum safe speed for which the tire may be used. The speed rating uses an alphabetic code with specific speeds associated with individual letters. Higher speeds are generally associated with letters toward the end of the alphabet. In our example, V indicates that the tire is certified for speeds up to 149 mph. Contrary to the assumption made by many consumers, the speed rating does not pertain to the tire's performance capabilities at high speeds. Rather, it is solely a measure of the tire's ability to withstand the heat generated by driving at high speeds. Tires with lower speed ratings will deteriorate faster as a result of heat if the vehicle on which they are used is frequently driven at high speeds. Not all tires are speed-rated, but if your car originally came with speed-rated tires, replacements should also be speed-rated. Following is a list of letters used in speed ratings and the speed in mph associated with each.

F = 50 mph
G = 56 mph
J = 62 mph
K = 68 mph
L = 75 mph
M = 81 mph
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

The tire code may contain one final element that is simply the manufacturer's name or a descriptive designation for the tire.

Professor Wizard

There will be a quiz tomorrow.

Those who fail to pass he quiz will have to trad in their Mustang and drive a Kia Sportage.

lostsoul

huh....

Orion_240

I remember learning all that tire stuff back in the 80's when I saw my first Lamborghini Countach and those insanely wide tires in the back! I'm still amazed at the very low profile tires I see the tuner kids driving on lately.

lostsoul

the codes are nice.. but I'm still trying to find out what the max size tire I can fit on back the my base GT rims .. I'm hearing 275s.. but its shaky and 255s in front..

DirtyGt

but you failed to list the actual reason for speed ratings. come on impress me.... why did they start rating tires in the first place????

don_w

Someone here posted a week or so ago (sorry, forgot who) that they had 275/50s with "room to spare". Unless I can find a good quality 285/50 (anyone know of one?), I'll probably go with the 275... but I'll stick to a 17" wheel.

don_w

It was Troden... https://mustangforums.com/m_470190/m.../tm.htm#470215

lostsoul

yea.. but he had 17 x 9 ... I thought stock was 17 x 8 ??

==DirtyGt== the reason for the speed ratings is that not all tires are made equil you dont want to hit a 140 in pep boys futura tires..hahaha..

don_w

lostsoul... have you checked out the tire company websites. There are many tables showing the rim size needed for various tire sizes.