Speed Rating

The speed rating of a tire is an important detain which is often overlooked and sometime forgotten when people shop for tires. Here we explain what it's all about and how it relates to load rating  ...



Often  people will include the speed rating and load rating of a tire when they are specifying a tire's size, other times people forget about it entirely.

The size is really something independent of the both the speed and load rating, but because the codes for all three are stamped on the sidewall together, they are often used together.

Speed and Load

In simple terms, the speed rating of a tire is the maximum speed at which the tire is designed to be operated at when carrying the load which is specified by the load rating.

At first glance you might wonder why speed and load are linked together, but when you realize that tires generate heat when they are being used because the tires are being constantly flexed as the wheel rotates, you may beging to see the connection.

Let's do a little thought experiment (or if you wish, get a ballon and do this for real).

speed rating baloon

First, imagine a round baloon sitting on a table. It would likely be nearly perfectly round but if you press down on it, the bottom part, which is on the table, becomes flat. The more you press -- that is the more weight you put on it-- the flatter it becomes on the bottom. This is causing it to flex.

That is the weight part. Now imagine pushing the baloon so that it "rolls" on the table and notice that the flat part on the bottom moves in that even though the baloon is rotating as you roll it, the flat part stays at the bottom, meaning that the rubber in the baloon is flexing also because of its rolling.

Flexing anything causes it to get hot if you do it fast enough and the rubber in a tire or baloon is no exception. The faster you go, the more you will flex the tire and with greater weight you are also flexing the tire more.

Heat + Rubber = Bad

The heat produced by flexing warms up the rubber compound which the tire is made from and heat is an enemy of that rubber. Apply enough heat to a tire and you will either melt it, or it will burn. How much heat, depends on the exact composition of the tire, but through testing it is possible to determine for each tire design what is the maximum combination of weight and speed that the tire will support before it begins to suffer weakening effects because of the heat. This is where the speed rating comes from.

After this is determined that tire is given a Speed Index and usually the car manufacturer will specify a certain speed index based on the maximum speed which the vehicle is capable of reaching.

The Speed Index code, together with a tire's Load Rating may be shown together in what is called the tire's "Service Description."
This is usually noted following the tire's size as in: 225/50R16 91H

Table of the Speed Ratings currently in use.

Symbol Miles per hour Km per hour Typical Use or Application
L 75 mph 120 km/h
Off-Road & Light Truck Tires
M 81 mph 130 km/h
N 87 mph 140km/h
Temporary Spare Tires
P 93 mph 150 km/h
Q 99 mph 160 km/h
Studless & Studdable Winter Tires
R 106 mph 170 km/h
H.D. Light Truck Tires
S 112 mph 180 km/h
Family Sedans & Vans
T 118 mph 190 km/h
Family Sedans & Vans
U 124 mph 200 km/h
H 130 mph 210 km/h
Sport Sedans & Coupes
V 149 mph 240 km/h
Sport Sedans, Coupes & Sports Cars
W 168 mph 270 km/h
Exotic Sports Cars
Y 186 mph 300 km/h
Exotic Sports Cars
Z +149 mph +240 km/h

At one time it was believed that no vehicle used on public roads would likely need a speed rating of greate than 149 mph, an all-inclusive top rating category Z was established to cover all tires which exceeded this speed capability.

Later two additional codes, W and Y, were added, but even where these are used, the Z rating still may be incorporated into the tire's size description --for example 225/50ZR16 91W -- where the true speed rating is still shown in the service description.

 

tireguy

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