Reading a Tire Sidewall

When shopping for tires, you need to know what the numbers and letters on your sidewall mean in order to select the appropriate replacement tire.

Tires usually follow a pattern can be illustrated by dissecting "Bridgestone Potenza RE040 205/50VR15 85V" like an earthworm in formaldehyde:

• Brand: Bridgestone. This is the company that manufactured the tire.

• Series: Potenza. Tire manufacturers often create a series of tires with somewhat similar handling and performance characteristics. For example, Bridgestone uses the Potenza name on many tires that may appeal to drivers who want a performance tire, and the Turanza name on many tires that are intended for less aggressive driving (or drivers with a less aggressive self-image).

• Model: RE040. This is a pretty specific identification of the general performance characteristics of this tire, regardless of its size. Tires in the Potenza series vary greatly in performance, but tires of the RE040 model have virtually identical construction, tread compound, design, and ride

• Width: 205. This number is the width of the tire in millimeters. This is the most important number to describe a tire's contact patch. For example, a 215-width, 17-inch tire may look great, but all things being equal, a 245-width, 15-inch tire will out-corner it every time, because the contact patch is wider.

• Aspect ratio: 50. This is the height of the sidewall from the rim to the tread, expressed as a percentageof the tread width. For example, if the tire is 205 mm wide, and its aspect ratio is 50, then the sidewall is about 102.5 mm tall (50 percent of 205 mm). Sidewall height is important for a number of reasons:
  • • As you move up to larger wheels, or down to smaller ones, a corresponding change needs to happen in the sidewall height of the tire in order for the rolling diameter of the wheel and tire combination to be as close to stock as possible. This will ensure the accuracy of your speedometer and prevent unwanted alignment changes.
    • The sidewall height affects the turn-in feel (the responsiveness you feel at the steering wheel) and the ride quality.
    • Lower aspect ratio (shorter sidewall) provides better turn-in response than a higher aspect ratio, but at the expense of less break-away warning and a more jarring ride — in extreme cases, even exposing the wheels to potential bending and breaking damage from potholes and other surface irregularities.
• Speed rating: VR (V-rated radial). In this case, V identifies a speed rating above 149 mph. It's almost impossible to find a public road in the U.S. where you can legally drive faster than any tire's rated speed, but speed ratings are a very important safety consideration when shopping for tires for motor sports and countries that allow high speeds on public roads. Table 1 lists speed ratings. R means the tire is a radial. Virtually every new tire is a radial, unless you have a special tire for a classic car or a racing car.

Table 1: Speed Rating Designations

• Load rating: 85V. In this case, 85 means the tire is rated for 1,135 pounds (see Table 2). Multiply this by the number of tires on the car, and you get a maximum safe loaded vehicle weight of 4,540 pounds (including people, luggage, a full tank of gas, and all the loose change under the seats). The last V repeats the V speed rating, listed in Table 1.

Table 2: Load Rating Designations If your tire isn't described by these two tables, then consult the tire manufacturer or a tire specialist, such as Tire Rack or Wheel Works, for further information.