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Tire Basics and Selection

What: Tires are categorized by their type of construction. The two main types are: bias ply and radial ply. They both share the same basic construction characteristics except for the orientation of the cord in the sidewall. The bias ply tire has the cords for the sidewall running from bead to bead on about a 45-degree angle. The radial ply tire has the cords for the sidewall running from bead to bead on a 90-degree angle. The kind, type and style of vehicle they are being installed upon dictate which of these types of tire construction you choose.

Tire Identification: Reading the sidewall. The sidewall of a tire is, in effect, an owner’s manual. The designations and classifications that appear on it identify common dimensions and standard test identification numbers. Being able to read the sidewall will help to better understand the performance standards of a tire.

Aspect Ratio: As the aspect ratio of a tire is part of most sizing systems, it is important to know what it is and what it means. A tire’s aspect ratio is the dimensional relationship of the tire’s section height to section width. The lower the aspect ratio, the shorter the sidewall, and the quicker the steering response. When the aspect ratio appears in the sizing of the tire, it precedes the tire construction designation, except in the Alpha-Numeric system.

Sizing Systems: Three sizing systems exist for passenger car tires today — P-Metric, European Metric and Alpha-Numeric. Each of these systems evolved from the first tire sizing system, the Numeric Sizing system, which is now obsolete. It was developed when all tires had the same aspect ratio, and it provided only the nominal cross section width of the tire and the rim diameter in inches. Here are examples that identify the three sizing systems that are commonly seen today.

P-Metric: Today most U.S. tire manufacturers build tires that conform to this system. It evolved in the 1970s to accommodate the introduction of small tires used on economy cars and is based on the metric system.

P—Passenger Car Tire
215—Section Width in Millimeters
65—Aspect Ratio
R—Radial Construction
15—Rim Diameter in Inches


European Metric: Essentially this system was a conversion of the Numeric System from inches to millimeters. Aspect ratio appears in the size designation in most cases where it is other than 82.

155—Cross Section Width in Millimeters
Aspect Ratio Assumed 82
S—Speed Rating
R—Radial Construction
13—Rim Diameter in Inches


185—Cross Section Width in Millimeters
70—Aspect Ratio
S—Speed Rating
R—Radial Construction
14—Rim Diameter in Inches


Alpha-Numeric: In the 1960s this load-based system evolved. The first letter of the designation identifies the load/size relationship of the tire. The letter can range from "A" to "N". The lower the letter the smaller the size and load-carrying capacity at a
given inflation pressure.


B—Load Capacity
R—Radial Construction
60—Aspect Ratio
13—Rim Diameter in Inches


Speed-Rating System: The speed rating of a tire indicates the speed category (or range of speed) at which the tire can carry a load under specified service conditions. A letter from A to Z symbolizes a tire’s certified speed rating. This rating system describes the top speed for which a tire is certified. It does not indicate the total performance capability of a tire.

What Tire To Choose: The selection should be determined by function. The tire chosen must be appropriate for the rim diameter and rim width of the wheel. It must be rated for the application and load to be carried. The height must not exceed the confines of the wheelwell and fender opening. The aspect ratio and height of the sidewall will have a direct effect on ride and handling. (I.e. short stiff sidewalls will help with improved cornering but make the ride firmer.) Wheels larger than necessary will require that tires with a lower aspect ratio be used in order to keep the tire height dictated by the confines of the wheel openings and proper gear ratios. Anything larger is chosen for the sake of style. See What Wheel to Choose.

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