The sidewall of a car's tire has important information that tells you almost everything you need to know about it. The best way to understand tire markings is to break down the letters and numbers in an example.
For our example, the numbers are:
P215/65R 15 95H
(P) Service Description
The service description may not always appear on the tire, but it is important to know how it can affect your vehicle. If there is a "P" on the sidewall, it stands for "passenger car." This refers to the U.S. (P-metric) method of tire sizing. "LT" stands for Light Truck, "ST" is for Special Trailer and "T" stands for Temporary, which is primarily used for small spare tires. If a tire does not have a "P" or another letter in front of the numbers, it is considered a "Euro-metric" tire. A Euro-metric tire conforms to the European tire specifications, and often carries a different load index than a comparably sized P-metric tire. We'll discuss load index in more detail below.
(215) Tire Width
The first number in this series refers to the tire's section width, or the distance from sidewall edge to sidewall edge, measured in millimeters up and over the tire's tread. Generally speaking, the larger this number is, the wider the tire will be.
(65) Aspect Ratio
This number is the tire's aspect ratio, or its section height compared to its section width. In this example, the section (or sidewall) height is 65 percent of the section width. This number can be indicative of a tire's purpose. Lower numbers, like 55 or less, mean a short sidewall for improved steering response and better overall handling.
(R) Internal Construction
The "R" refers to radial construction, which has been the industry standard in passenger-car tires for more than 25 years. Prior to radial tires, most cars came with bias-ply tires, which had a crude construction that made for poor handling. Bias-ply tires (which use a "B" for their description) are still used for certain truck applications.
(15) Rim Diameter
This is the wheel (or rim) diameter, in inches, for which the tire was sized. Pay particular attention to this number if you plan on upgrading your wheel size. If your wheel diameter changes, you'll have to purchase a new set of tires that matches this new diameter.
(95) Load Index
A tire's load index is a measurement of how much weight each tire is designed to support. The larger the number, the higher the load capacity. This is one of the most important numbers on your tire. To find out what "95" means, you have to look it up on a Load-Carrying Capacity Per Tire chart. Ninety-five indicates a maximum weight of 1,521 pounds. Remember that this is per tire, which means you have to multiply by four to get the total capacity for a complete set of tires. If the vehicle has its original tires, you can just refer to the doorjamb, which lists the maximum cargo capacity with passengers.
Some vehicles are equipped with "XL" tires. It doesn't mean that they're extra large, but it does mean that they are extra-load tires. The load index on these tires is much higher than a standard-load tire, which is why it is important to replace an XL tire with another XL tire.
Earlier, we discussed "P-metric" and "Euro-metric" sizing, and it's the difference in their load rating that can lead to confusion and potential trouble. For a given size, P-metric tires will have a load index that is one or two points lower than corresponding Euro-metric tires. So if your car came with Euro-metric tires, don't replace them with P-metric tires. You can, however, replace P-metric tires with equivalent-size Euro-metric ones because you gain load capacity that way.
Why is this important? Generally speaking, you don't want your replacement tires to have a lower load index number than the originals (again, as indicated by the driver's doorjamb or the owner's manual), particularly with high-capacity vehicles that ride on smallish tires, such as minivans.
Also, optional large-diameter wheels with lower-profile tires tend to have less load-carrying capacity because they contain less air. And it is the volume of air inside the tire, not the rubber itself or the wheel material that shoulders the load. The load index is especially important when shopping for a tire online, since some retailers do not specify whether a tire is P-metric or not.
(H) Speed Rating
The speed rating is a measurement of the speed at which the tire is designed to run for extended periods. An "H" speed rating signifies that this tire can be run safely at speeds of up to 130 mph for extended periods. Will it explode if it goes to 140? Not immediately. But it might, if it is run at that speed for an extended time.
Here is a complete list of the various tire speed ratings, and their associated letters:
S = 112 mph
T = 118 mph
U = 124 mph
H = 130 mph
V = 149 mph
*Z = Over 149 mph
*W = 168 mph
*Y = 186 mph
*(Y) = Over 186 mph
*The "Z" rating used to be the highest rating for tires having a maximum speed capability greater than 149 mph. But as tire technology improved, it ultimately split into the "W" and "Y" ratings. A "ZR" may sometimes appear in the size designation, as a sort of nod to the prior rating, but it will also be used in conjunction with a W or a Y. When a Y rating is enclosed in parentheses, it means that the tire is capable of speeds in excess of 186 mph.
Additional Information on Your Tires
The DOT code is used by the Department of Transportation (DOT) to track tire production for recall purposes. If a tire proves to be defective, this number helps keep track of where these tires ended up so buyers can be notified of the problem. At the end of the DOT code you'll find a four-digit number. This is the manufacturing date of the tire. The first two digits stand for the week; the other two are the year. For example, if your tire had "1613" listed, it was manufactured on the 16th week of 2013.
If you come across a three-digit number, you have a tire that was manufactured before 2000. A DOT tire code of "127" indicates the tire was made on the 12th week of the seventh year of the decade. But it's difficult to know whether that was 1997 or even 1987. According to tirerack.com, some tires produced in the 1990s may have a small triangle following the DOT number to identify the decade. But any tire that has a three-digit code is history. Tire experts recommend that you replace tires that are six or more years old, regardless of their tread depth.
Sometimes the DOT number will be located on the inside of the tire. In this case, you can either jack up the car to inspect it, or check with your local mechanic or tire shop. You should also make a habit of checking the manufacturing date on your spare tire as well.
Maximum Air Pressure
This number refers to the maximum amount of air you can put in a tire before you harm it. It is not the recommended tire pressure; that number can be found in your owner's manual and on the doorjamb.
A traction rating can also be found on the sidewall of all modern tires. It can be represented as AA, A, B or C. This is a rating of a tire's traction when tested for straight-line braking on a wet surface. For this rating, AA signifies the best traction performance and C indicates the worst.
The temperature rating refers to the ability of the tire to withstand heat at high speeds. The ratings, from best to worst, are: A, B and C.
Finally, you might find the word "TREADWEAR" on the sidewall followed by a number like 120 or 180. This is a rating of the tread's durability, as tested against an industry standard. The reference number is 100, so a tire with a treadwear rating of 200 has a predicted tread life that's twice as long as the industry standard, while a rating of 80 means a predicted tread life that's only 80 percent as long as the industry standard.
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