Purcell: We'll keep you moving

Reading the Sidewall of a Tire

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 that lists Brand, Series, Model, Width, Aspect Ratio, and Speed Rating.

Series - Tire manufacturers often create a series of tires with somewhat similar handling and performance characteristics.  

Model -The specific identification of the general performance characteristics of a tire, regardless of its size.

Width - This number is the width of the tire in millimeters. This is the most important number to describe a tire's contact patch.

Aspect ratio - This is the height of the sidewall from the rim to the tread, expressed as a percentage of the tread width.

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 - 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.