# Tire Tech

(Lea en español)

Using a P195/60R15 87S tire size as our example, the 87S at the end of the size represents the tire's service description. A service description identifies the tire's load index and speed rating. Service descriptions are required on all speed rated (except for Z-speed rated) tires manufactured since 1991.

The first two digits (87S) represent the tire's load index and are followed by a single letter (87S) identifying the tire's speed rating.

"Using a P195/60R15 87S tire size as our example, the 87S at the end of the size represents the tire's service description. A service description identifies the tire's load index and speed rating."

P195/60R15 87S - The load index (87) is the tire size's assigned numerical value used to compare relative load carrying capabilities. In the case of our example, the 87 identifies the tire's ability to carry approximately 1,201 pounds.

The higher the tire's load index number, the greater its load carrying capacity.

89 = 1,279 pounds
88 = 1,235 pounds
87 = 1,201 pounds
86 = 1,168 pounds
85 = 1,135 pounds

A tire with a higher load index than that of the Original Equipment tire indicates an increase in load capacity. A tire with a load index equal to that of the Original Equipment tire indicates an equivalent load capacity. A tire with a lower load index than the Original Equipment tire indicates the tire does not equal the load capacity of the original.

Typically, the load indexes of the tires used on passenger cars and light trucks range from 70 to 126.

70 739 335 89 1279 580 108 2205 1000
71 761 345 90 1323 600 109 2271 1030
72 783 355 91 1356 615 110 2337 1060
73 805 365 92 1389 630 111 2403 1090
74 827 375 93 1433 650 112 2469 1120
75 853 387 94 1477 670 113 2535 1150
76 882 400 95 1521 690 114 2601 1180
77 908 412 96 1565 710 115 2679 1215
78 937 425 97 1609 730 116 2756 1250
79 963 437 98 1653 750 117 2833 1285
80 992 450 99 1709 775 118 2910 1320
81 1019 462 100 1764 800 119 2998 1360
82 1047 475 101 1819 825 120 3086 1400
83 1074 487 102 1874 850 121 3197 1450
84 1102 500 103 1929 875 122 3307 1500
85 1135 515 104 1984 900 123 3417 1550
86 1168 530 105 2039 925 124 3527 1600
87 1201 545 106 2094 950 125 3638 1650
88 1235 560 107 2149 975 126 3748 1700

When looking at light truck (LT) or newer Special Trailer Service (ST) tires, there are two load indexes branded on the sidewall, separated by a forward slash. Using an LT235/75R15 104/101S Load Range C tire as an example, the load index is 104/101. 104 corresponds to 1,984 pounds, and 101 corresponds to 1,819 pounds. So what is the true load carrying capacity of the tire? The answer changes depending on the situation in which the tire is being used.

Since LT tires are commonly used on trucks with dual rear wheels, they are branded with two load indexes. The first number indicates the load carrying capacity if the tire is installed on a truck with a single-wheel rear axle, and the second number applies when the tire is used in a dual rear application.

Though it may seem counterintuitive that a tire is rated to carry less weight when working in tandem with another tire in the dual pair, the purpose is to build in additional reserve capacity should one of the two tires fail, leaving the sole remaining tire to carry the load normally handled by two tires.

### Speed Rating

In Germany some highways do not have speed limits and high speed driving is permitted. Speed ratings were established to match the speed capability of tires with the top speed capability of the vehicles to which they are applied. Speed ratings are established in kilometers per hour and subsequently converted to miles per hour (which explains why speed ratings appear established at "unusual" mile per hour increments). Despite the tire manufacturers' ability to manufacture tires capable of high speeds, none of them recommend the use of their products in excess of legal speed limits. The maximum operating speed of a vehicle must be limited to the lowest speed rated tire on the vehicle.

Speed ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire's required speed has been met.

It is important to note that speed ratings only apply to tires that have not been damaged, altered, under-inflated or overloaded. Additionally, most tire manufacturers maintain that a tire that has been cut or punctured no longer retains the tire manufacturer's original speed rating, even after being repaired because the tire manufacturer can't control the quality of the repair.

Over the years, tire speed rating symbols have been marked on tires in any of three ways shown in the following examples:

 225/50SR16 225/50SR16 89S or 225/50R16 89S

Each of these was an acceptable method of identifying speed ratings.

Early tires had their speed rating symbol shown "within" the tire size, such as 225/50SR16. Tires using this type of branding were not to have been produced after 1991.

 225/50SR16 112 mph, 180 km/h 225/50HR16 130, 210 km/h 225/50VR16 in excess of 130 mph, 210 km/h

Beginning in 1991, the speed symbol denoting a fixed maximum speed capability of new tires must be shown only in the speed rating portion of the tire's service description, such as 225/50R16 89S. The most common tire speed rating symbols, maximum speeds and typical applications are shown below:

 L 75 mph 120 km/h Off-Road & Light Truck Tires M 81 mph 130 km/h Temporary Spare Tires N 87 mph 140km/h 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

When Z-speed rated tires were first introduced, they were thought to reflect the highest tire speed rating that would ever be required, in excess of 240 km/h or 149 mph. While Z-speed rated tires are capable of speeds in excess of 149 mph, how far above 149 mph was not identified. That ultimately caused the automotive industry to add W- and Y-speed ratings to identify the tires that meet the needs of vehicles that have extremely high top-speed capabilities.

 W 168 mph 270 km/h Exotic Sports Cars Y 186 mph 300 km/h Exotic Sports Cars

While a Z-speed rating still often appears in the tire size designation of these tires, such as 225/50ZR16 91W, the Z in the size signifies a maximum speed capability in excess of 149 mph, 240 km/h; the W in the service description indicates the tire's 168 mph, 270 km/h maximum speed.

 225/50ZR16 in excess of 149 mph, 240 km/h 205/45ZR17 88W 168 mph, 270 km/h 285/35ZR19 99Y 186 mph, 300 km/h

When the Y-speed rating indicated in a service description is enclosed in parentheses, such as 285/35ZR19 (99Y), the top speed of the tire has been tested in excess of 186 mph, 300 km/h indicated by the service description as shown below:

 285/35ZR19 99Y 186 mph, 300 km/h 285/35ZR19 (99Y) in excess of 186 mph, 300 km/h

As vehicles have increased their top speeds into Autobahn-only ranges, the tire speed ratings have evolved to better identify the tire's capability, allowing drivers to match the speed of their tires with the top speed of their vehicle.

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