GTO JUDGE

How to "read" your tires to know if they're too old.
By Jan Kuderna

The useful life of a tire is only five to seven years. For cars and trucks driven every day, the tread usually wears out in less than five years. For RVs that sit for a good part of the year, five years can pass with a lot of tread still left on the tire.

Although you may not want to replace what looks like a perfectly good tire, riding on tires more than five years old greatly increases the risk of a blowout.

Date Codes: Every tire has a date code stamped on the sidewall, which gives the date that the tire was manufactured. They look something like this: DOT PDHH MLOR 3403. The date code can be on either side of the tire, so you may have to crawl underneath the rig and look on the inward facing side. The date code always starts with the letters DOT and ends with a 3 or 4 digit number. That last number is the date code, which tells you when the tire was manufactured. The first two numbers indicate the week (out of 52) and the last one or two digits indicate the year. For instance, 3403 means the 34th week of 2003, or the last week in August 2003. Starting with the year 2000, the date codes have two digits for the year, prior to that, only one. A date code of 079 would indicate the seventh week of 1999, or the third week of February 1999.

Tires deteriorate with age, even when sitting on a shelf, so always ask to see the date code when you purchase new tires and insist on tires manufactured within the last few months. The tire dealer may give you a funny look because most consumers don't know about date codes.

Tire Size Designations: That jumble of letters and numbers on the sidewall of the tire is the tire size designation. The first letters indicate the type of tire: P for passenger car, LT for light truck, and ST for special trailer. Bus and medium-duty truck tires have no such designation. The next number is the width of the tire, given in millimeters, followed by a slash. The number following the slash is the ratio of width to section height (only important to tire engineers) followed by a letter: R for radial ply or D for diagonal or bias ply. It ends with a number which gives the inside diameter of the tire in inches. A tire with the designation ST225/75R15 is a special trailer tire that is 225 millimeters wide with a width to section height ratio of 75. It is a radial ply tire that will be mounted on a 15-inch wheel.

Load Range: The load range of a tire is indicated by a letter, A through E, and is stamped on the sidewall of the tire. Tire charts, available from any tire dealer, have these letters in parentheses after some of the tire load limits. The letters are placed next to the maximum weight for that load range.

Which Type of Tire to Use
Tires are engineered specifically for different types of vehicles. Passenger car tires are designed to provide a soft ride and grip the road during turns and adverse weather. Light truck tires have stiffer sidewalls in order to carry heavier loads, but also are engineered for safe handling and road gripping ability. Trailer tires, on the other hand, are designed to give a soft ride and to slide sideways or scrub the road while cornering. Because of these differences, never put light truck tires on a trailer. Some people think that if the tire is good enough for a truck it must be good enough for a trailer, but this is a fallacy. Light truck tires are not engineered for the unique stresses of trailering.



How to Read Speed Rating, Load Index & Service Descriptions

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.

Load Index

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 tires 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 110.
Load Index Pounds Kilograms
Load Index Pounds Kilograms
71 761 345 91 1356 615
72 783 355 92 1389 630
73 805 365 93 1433 650
74 827 375 94 1477 670
75 853 387 95 1521 690
76 882 400 96 1565 710
77 908 412 97 1609 730
78 937 425 98 1653 750
79 963 437 99 1709 775
80 992 450 100 1764 800
81 1019 462 101 1819 825
82 1047 475 102 1874 850
83 1074 487 103 1929 875
84 1102 500 104 1984 900
85 1135 515 105 2039 925
86 1168 530 106 2094 950
87 1201 545 107 2149 975
88 1235 560 108 2205 1000
89 1279 580 109 2271 1030
90 1323 600 110 2337 1060

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 manufacturer's ability to manufacturer tires capable of high speeds, none of them recommend the use of their products in excess of legal speed limits.

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:
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

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 new 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

Most recently, 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 tires capability, allowing drivers to match the speed of their tires with the top speed of their vehicle.

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