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Wheels and Tires:
Take the mystery out of buying new tires!
Learn to read the fine print
Thanks to recent tire advances, your tires last a lot longer than they used to. However, even with proper maintenance, tires do wear out and need to be replaced. If you've ever made a trip to a tire store, you probably were inundated with endless tire options. How can you find the right tire to fit your driving habits?
Finding the right size
First off, you'll need to know your tire size. If you look on the sidewall of your current tire, you'll see something like 'P215/75 R15.' That's the size.
All manufacturers use a uniform P-Metric Scale to rate tire size:
Deciphering the Code
Unless you're already familiar with the P Metric Scale, it may be difficult to translate our example tire's code. But with just that little information, an auto pro can install the right tire every time. In fact, in just seconds they could tell you the use; tire width, the tire ratio, type and even the diameter. So let's break it down so you'll know what to look for the next time you buy tires.
 
P 215/ 75 R 15
Tire use: The first letter indicates the category or type of use. Nearly all car tires will be identified with a "P" for passenger.
P=Passenger Tire
C=Commercial Tire
T=Temporary Tire
P 215/ 75 R 15
Tire Width The first number is the tire width, measured in millimeters, from sidewall to sidewall. To convert to inches, divide by 25.4. Therefore, our tire above is about 8.5 inches wide.
P 215/ 75 R 15
Tire Ratio The second number is the aspect ratio. In other words, the ratio of sidewall height to width. Aspect ratio determines the profile, or how 'tall' your tire will look. It also determines the contact patch-or footprint-that your tire makes on the road. To find the height of one sidewall, multiply the tire width by the aspect ratio. For our tire above, one sidewall is about 6.3 inches tall.
P 215/ 75 R 15
Tire Type Almost all tires will have an 'R' for radial.
P 215/ 75 R 15
Wheel Diameter This is the measurement of the wheel in inches. This measurement corresponds to the rim diameter.
If you take a look at your sidewall, you'll see numerous ratings printed across the side. This is part of the Uniform Tire Quality Grading System, a standardized classification designed to rate a tire's durability. Some important factors include:
Maximum pressure and load. It's important to know how much your tires can take when you are hauling a heavy load. However, this pressure should not be used for daily driving. Your car will have a small sticker on the driver's doorpost, inside the fuel door or inside the glove box that contains the proper pressure for the recommended tire size.
Traction rating measures the amount of friction produced on wet asphalt when you slam on your brakes. An 'A' grade is better than a 'B' grade, but note that this rating does not indicate wet cornering ability.
Temperature resistance measures how long a tire can withstand being rolled against a steel wheel at increasing speeds without disintegrating. As in the traction test, the highest grade is 'A.'
Tread wear grade comes in multiples of 10-a higher number indicating a longer tread life. However, tread wear tests are done with fewer specifications than traction and temperature ratings, so it would be a mistake to compare tread wear ratings across brands.
Other Markings
DOT Code
The DOT Code is an alphanumeric character sequence molded into the sidewall of the tire for purposes of tire identification. The DOT Code is mandated by the US Department of Transportation. The DOT Code is useful in identifying tires in a product recall.
The DOT Code begins with the letters "DOT" followed by a two numbers or letters plant code that identifies where it was manufactured. The last four numbers represent the week and year the tire was built. A three-digit code was used for tires manufactured before the year 2000. For example, 178 means it was manufactured in the 17th week of 8th year of the decade. In this case it means 1988. For tires manufactured in the 1990s, the same code holds true, but there is a little triangle (Δ) after the DOT code. Thus, a tire manufactured in the 17th week of 1998 would have the code 178Δ. After 2000, the code was switched to a 4-digit code. Same rules apply, so for example, 3003 means the tire was manufactured in the 30th week of 2003.
E-mark
All tires sold for road use in Europe after July 1997 must carry an E-mark. The mark itself is either an upper case "E" or lower case "e" - followed by a number in a circle or rectangle, followed by a further number. An (upper case) "E" indicates that the tire is certified to comply with the dimensional, performance and marking requirements of ECE regulation 30. A (lower case) "e" indicates that the tire is certified to comply with the dimensional, performance and marking requirements of Directive 92/33/EEC. The number in the circle or rectangle denotes the country code of the government that granted the type approval. The last number outside the circle or rectangle is the number of the type approval certificate issued for that particular tire size and type.
Mold serial number
Tire manufacturers usually embed a mold serial number into the sidewall area of the mold, so that the tire, once molded, can be traced back to the mold of original manufacture. |
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 Articles
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Article Pages
1. An Introduction to Wheels and Tires
2. Tires
4. Manipulating The System
5. Valves
6. Tire Pressure Monitoring Systems (TPMS)
7. Tire Inspection
8. Tire Wear Guide
9. Tire Rotation
10. Wheel Assembly
11. Wheel And Rim Construction
Hot Topics
-> 4 Things You Can Start Doing Today To Extend The Life Of Your Tires
-> New Tires Guide
-> Tire Safety Tips
-> What Does It Mean When My Wheels Are Imbalanced?
-> Wheel Balancing
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