Replacing Your Original Factory Tires 101

If you are not satisfied with the way your car drives, get new tires of the same size and characteristics. Look at the side of the worn-out tire and make sure that your new tires have the same key dimensions and ratings.

Match the size and tire ratings!

  • Tire Type
  • Tire Width
  • Aspect Ratio
  • Construction
  • Wheel Diameter
  • Load Index
  • Speed Rating

Tire Type

This designates the type of vehicle the tire fits. P is for passenger metric. Other letters are LT (for light truck), T (for temporary spare) and ST (for special trailers). If your tire has no letter, it signifies that your tire is a euro "metric" size.

Tire Width

Also called Section Width, this is the width of the tire (or thickness) in millimeters, if measured from a tire's widest point of its outer sidewall to the widest point of its inner sidewall. Why millimeters? It originated in Europe, which uses the metric system.

Aspect Ratio

This identifies the tire's aspect ratio, which is the relationship of the tire's sidewall height to the tire's width. In this example, the sidewall height of the tire is 55% of its width. The lower the ratio, the smaller the sidewall height, which means better cornering, but a rougher ride.

Construction

This is the tire's internal construction, which is "radial." Almost every tire on the road has radial construction, which means the cords of the carcass plies inside the tire "radiate" directly across from one side of the tire to the other. Other letters used are D, for diagonal construction, and B, for belted.

Wheel Diameter

This number (in inches) indicates that the tire is designed to fit on a wheel with a 18-inch diameter.

Load Index

This indicates how much weight the tire is certified to carry at maximum safe inflation. It doesn't mean 97 pounds, because it's actually an assigned value that corresponds with its "actual" load capacity found on a load index chart. If you look up 97 on the chart, you'll find 1,609 pounds.

Speed Rating

This indicates the maximum safe speed at which a tire is certified to carry a load under specified conditions. Speed ratings range from A (lowest) to Y (highest), with one exception: H falls between U and V.

The tire sidewall not only proudly displays the name of the manufacturer (like Michellin, Nitto, or Toyo), it also features three sets of codes that give you key information about the tire.

  • The first set of codes (2) tells you what kind of vehicle the tire is made for, the measurements of the tire and the load- and speed ratings. Here is how to interpret the numbers:
  • The letter (a) tells you if the tire is made for passenger cars (P), for light trucks (LT), if it is a tire for a temporary spare (T) that can only be used to drive the vehicle to the nearest tire shop, or if it is a special trailer tire (ST). If there is no letter, it means that you have a "euro" metric size tire in front of you.
  • The number in front of the slash (3) is the width of the tire in mm, measured from the widest point on the outer side of the sidewall to the widest point on the inner side of the opposite
  • The number after the slash (4) describes the profile of the tire, i.e. the relationship between the height of the sidewall and the width of the tire. In this case the height of the sidewall is 55% of the tire width. Low numbers mean better cornering power and rougher ride.
  • The letter (5) describes the construction of the tire. Just about all modern road tires are marked "R", designating radial construction. Other types are diagonal (D) and belted (B).
  • The next number (6) tells you the diameter (in inches) of the wheel that the tires is designed for. In this case the tire fits on 18" wheels.
  • The next number (7) tells you the load rating of the tire. This is a code, where a higher number corresponds with a higher load rating. Load indexes for tires used on passenger cars and light trucks range from 70 to 130.
  • Finally, the letter (8) shows you the speed and wear rating for the tire.

If all these numbers and letters are the same on your new tires as on the old worn out tires, your new tires will fit.

Don't buy old tires!

The second set of codes tells you that the tire complies with the DOT standards and are legal to use. The first code after the DOT insignia is the DOT code for the specific tire. The next set of codes tells you in which plant the tire is made. The final four numbers are very important as they tell you when the tire was manufactured.* The two first numbers of this group tells you the week of the year and the last two numbers tell you the year of manufacture.**

This is very important information, as tires must be fresh to perform properly. Tires dry out with age. This makes them harder and reduces their traction, just like old erasers get hard and useless. Old tires are dangerous even if they still have a good tread depth. Always make sure that your new tires are fresh! Don't buy tires that are more than a year old!

Quality markings to guide you

Quality Markings To Guide You

The third set of codes that can be found on all new tires describes the tire's rating for treadwear, traction and temperature dissipation. This is a federally mandated rating done according to standards set by the National Highway Traffic Safety Administration, NHTSA. This is called Uniform Tire Quality Grading, UTQG, and it is intended to keep unsafe tires off the roads.

  • The treadwear rating is only comparable within one manufacturer's lines of tires. The base value is 100, and a tire with a 200 rating should last twice as long as a 100-rated tire, all other things being equal.
  • The traction ratings are AA, A, B and C, with AA being the highest rating. Tires with a rating below C should not be used on public roads.
  • The temperature ratings are A, B and C. Tires rated below C are not suitable for use on public roads.

Writing on tirewalls can tell you more

In addition to the above information, which has to be molded into every tire, manufacturers provide even more information in the tirewall lettering.
All All-Season tires are marked M/S, M+S or M&S. The letters stand for Mud and Snow, which shows that the tire meets the standards set for mud and snow tires by the Rubber Manufacturers Association. These tires are good enough for places where occasional snow, but for months of winter weather you are better off with separate sets of tires for winter and summer conditions.

If you want to fit tires that can increase your gas mileage, you can look for marks that designate energy efficiency. There is no standard for this, so each manufacturer has its own energy efficiency markings. Modern energy efficient tires perform as well or better than regular tires, so if you don't drive on low roll resistance and energy efficient tires already, you should consider replacing them with energy efficient tires. They will save you money, save the environment from unnecessary emissions and contribute to the American trade balance by reducing the need for imported oil.

*If you don't see a complete set of DOT codes on the tire wall, look on the other side of the tire. The date code has to be changed in every tire mold every week, and by displaying it on only one sidewall manufacturers save 50% of that work.

**Tires made before 2000 only display three digits for the date of manufacture. The first two designate the week and the last digit the year of manufacture. Never buy a tire with a 3-digit code for the date of manufacture! That tire is too old!


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