Tires carry a wealth of information on their sidewalls; unfortunately, it's all in code. Most of it is of no concern to many owners, but it can be valuable for those who want to know the seasons for which their tires are intended, those who plan to replace their tires, or new-car buyers who are given a choice of sizes and types.
P simply means this is a passenger-car tire. Other codes include T (temporary, as in a spare tire), LT (light truck) and C (commercial). Not all tires include this letter.
205 (the first number) is the tire's width in millimeters.
55 (the number following the slash) is a measure of the tire's profile -- the height of its sidewall relative to its width -- expressed as the sidewall's aspect ratio in a percentage. In this example, the sidewall height is 55 percent of the tire's width, which is 205 mm. The number is known as the tire's series; a sporty tire with shorter sidewalls would be considered a lower-series tire. Off-road tires tend to be higher-series, designed to absorb the impact from rough surfaces.
R means it is a radial tire, which describes the way the underlying layers, or plies, are laid out. The radial design is now so pervasive that the R designation is superfluous -- though it conveniently separates the series and diameter numbers. On some performance tires, an additional letter precedes the R: A ZR designation means the tire's maximum speed rating is above 149 mph (see the key below). This is another unnecessary character; all tires provide a letter designation in the service description, explained below. It follows the size code, sometimes in smaller type.
Speed Ratings Letter / Max. MP
*Z is an open-ended rating that means the tire's maximum speed is, at minimum, 150 mph.
16 (the last number) is the diameter, in inches, of the rim on which the tire fits.
89H is the service description. Separated from the main code, it represents the tire's load and speed ratings. This tire's 89 load index represents 1,279 pounds (per tire), and the speed rating of H represents 130 mph. The speed rating is less important in the U.S., where the speed limits are below even the lowest listed rating (N, or 87 mph). In other countries -- and our own, when no one's looking -- people are known to drive 100 mph and above.
M+S stands for mud and snow, indicating that this is an all-season tire. It's not the clearest of the specs, because there aren't other designations for summer and winter tires: Summer tires simply lack the M+S, and winter tires are labeled M+S and add an icon of a mountain plastered with a giant snowflake. (S, AS and W would be better, but what can you do?) Uniform Tire Quality Grade The following three codes, which appear on the sidewall, typically closer to the tire's tread, are part of the National Highway Traffic Safety Administration's uniform tire-quality grading system. Unfortunately, it's not all that uniform: It appears on all-season and summer tires, but not on snow tires, light-truck tires or spares. Further, the rating comes from the manufacturers, whose results might be consistent from one of their products to the next, but whose universality is questionable.
TREADWEAR 300 TRACTION A TEMPERATURE A
Treadwear: Uses numbers from 100 to about 700. Theoretically, a tire rated 150 would have a 50 percent longer lifespan than a tire rated 100 if used in the same conditions -- driver, vehicle and roads. Soft summer tires tend to have lower treadwear ratings than all-season tires. Winter tires aren't rated, but their soft compounds make for quick wear when driven off snow and ice.
The uniform tire quality grade is located on the sidewall near the tread. Traction: Uses AA, A, B and C, with AA being the best traction on a wet road. The traction measured is straight-line acceleration and braking. This is not a measure of cornering grip or performance on dry surfaces.
Temperature: Uses letter grades A, B and C, with A representing the best resistance to heat buildup and C the least. The friction of a tire on pavement generates heat, and too much heat degrades high-speed performance and can accelerate aging and failure. C is the lowest permissible rating. Temperature ratings correlate to speed, with C representing 85-100 mph, B representing 100-115 mph, and A covering speeds above 115 mph.
Notes: Arguably, the temperature rating -- like the "Z" speed rating -- is redundant, because it's tied to a speed rating that's presented with greater specificity in the service description. However, it's wise not to buy replacement tires with a rating lower than the originals', and it's a lot easier to match one of three temp ratings than it would be the 11 different speed ratings in the service description.
It's impossible to translate treadwear ratings to miles because where and how people drive plays a critical role. That said, if you think your tires wear out too quickly, consider a higher rating. The same goes for traction, but understand that changes in one characteristic typically change others, be it treadwear, noise, ride quality or price.
Tire Age/Date of Manufacture
Unlike the other, more-prominent specs, the Department of Transportation's Tire Identification Number is of interest to tire owners and buyers alike -- mainly because it includes the week and year of the tire's manufacture, and thus its age. Tire age, not just wear, has become a safety concern.
Found close to the rim, the TIN consists of a series of 10-12 letters and numbers that -- like a vehicle identification number -- are of little use to a consumer. But the last four numbers represent the birth week and year of any tire built since 2000.
DOT H25R YC24 4305
The tire above was manufactured in the 43rd week of 2005. Tires built before 2000 used a three-digit code, where the year was reduced to a single digit. If the tire above were from 1997 rather than 2005, its code would be 437. Would 437 also be the code for a tire made in the 43rd week of 1987? Unfortunately, yes, it would -- and that's why four digits are now used.
Recently, the effect of tire age on safety has become a concern. Unfortunately, climates and how well a tire is cared for affect its aging, so there are no universal rules. Most tires wear out before age becomes an issue, but some automakers -- mainly German ones -- recommend replacement after six years in service, regardless of wear. Most automakers agree that 10 years is the maximum safe lifespan for any tire, including a spare that has never been used. When buying new tires, it's wise to check their birth date. Treat them like milk: Your family might finish a gallon before its expiration date, but then again, it might not. Why take the chance? For the same money, you should get the freshest milk -- or tires -- you can find. You might wear the tires out long before they're due for retirement, but why buy a set whose clock is already ticking?
What's Not on the Sidewall: The Proper Inflation Pressure
If you look closely enough, you'll probably find a maximum-pressure figure on a tire's sidewall. Don't do it. This is not the recommended pressure for you to use; it's the maximum rating for the tire, irrespective of the vehicle on which it's used. Invariably, it's dramatically higher than the pressure you should be using.
New cars present the proper inflation pressures on the driver's doorjamb, as required by law. For more information, see Tire Maintenance.