Tire Speed Rating and Load Index

Understanding the Service Description, Load Index and Speed Rating

The Service Description is the alphanumeric code that appears at the end of the tire size (after the rim diameter) such as in this example below:

P225/45R17 91H

For the purpose of clarity, the Service Description is shown in red. It is comprised of two parts, the Load Index and the Tire Speed Rating.

Tire Load Index

The Load Index is the two or three digit number that appears first in the Service Description. It is the measure of a tires load capacity when inflated to its maximum inflation pressure (listed on the tire sidewall). A higher load index number indicates a greater load capacity. The tire load index chart below shows the different load index numbers along with their corresponding load capacities in both pounds and kilograms.

In the example above, the load index of 91 corresponds to a maximum load capacity of 1,356 lbs (615 kg). The total load capacity can be obtained by multiplying this number by four:

Load index (91) = 1,356 lbs. X 4 = 5,424 lbs. (2,460 kg)

Therefore, a set of four tires, each with a load index of 91 would be capable of carrying a maximum load of 5,424 pounds assuming each tire were inflated to its maximum rated inflation pressure.

As a general rule of thumb, when selecting a replacement tire it is best to select a tire with a load index equal to or greater than that of the vehicles original tire. If you do not know the load index of your vehicles original tires, be sure to consult your vehicles tire placard. The tire placard is attached to the vehicle and is typically located on the door jamb, door post, glove box door or fuel filler door.

Tire Speed Rating

The letter that appears in the Service Description of the tire is known as the Speed Rating. Simply stated, the Speed Rating of a tire is the maximum operating speed that the tire can sustain for a specified period of time. This rating is the result of laboratory testing under controlled conditions. It is therefore important to note that this testing does not account for under-inflation, tire damage, vehicle handling characteristics, or road conditions - all of which can lead to tire failure or loss of control at speeds lower than those indicated by the speed rating. Furthermore, since the tire speed rating on tires often far exceed the legal speed limit in most cases, it is not regarded as a safe driving speed for the vehicle itself.

Below you will find a tire speed rating chart that shows some common speed ratings along with their corresponding maximum speeds:

Sometimes, the speed rating appears within the tire size itself, and not as part of the service description such as in this example:

225/45HR17

This is an older nomenclature that was used on tires prior to 1991. With the exception of Z rated tires (see next paragraph), all tires produced since 1991 have the speed rating listed within the service description. In spite of this fact, you may find tire retailers that still use this older sizing convention, as it is familiar to many people.

For all non light truck tires manufactured after 1991, the nomenclature that you will see most often will look like this:

225/45R17 91H

Note that the speed rating (H) no longer appears in the tire size, but instead is shown in the service description

Z Speed Rating

A Z speed rating indicates that a tire has been speed tested in excess of 149 mph (240 km/h). The obvious question that next comes to mind is how far in excess of 149mph has this tire been tested?. Unfortunately, this question cannot be answered by the Z speed rating alone.

As mentioned in the previous section, in many cases, Z rated tires still show their speed rating within the tire size as shown below:

225/45ZR17 91Y

However, you will notice that although the Z rating appears within the tire size, there is still a service description at the end of the tire size, 91Y. The Y rating means that this particular tire has been tested for speeds up to 186 mph. So, while the Z speed rating it is somewhat noteworthy, it is more important to note the actual speed rating in the service description.