How to Understand Car Tire Sizes
Observing tire and vehicle load limits, maintaining appropriate tire pressure, installing correctly sized tires, avoiding road hazards and regularly inspecting tires all maximize automobile tire life. Proper tire maintenance offers improved vehicle handling, ameliorated fuel economy and increased tire life while decreasing accidents.
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Service Types
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Tire sizes generally begin with a letter or group of letters which identify the tire's appropriate vehicle or service type. When a tire size begins with a "P," it indicates the tire was designed for passenger vehicles. A "T" tire size designates the tire as a "temporary spare" designed for limited use until a damaged tire can be replaced. "LT" tire size stands for "Light Truck." If a tire size begins with "ST," the tire is a "Special Trailer Service" size made for use only on boat, car or utility trailers.
Width, Ratio and Construction
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After the service type, a three-digit number identifies the tire tread or section width, measured in millimeters. Immediately following the slash after section width is a two-digit number signifying the tire's profile or aspect ratio, which represents the tire height percentage over the rim compared to its width. Next is a letter to identify the tire's internal construction; commonly radial (R), diagonal/bias (D) or belted bias (B).
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Diameter and Service
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Two-digit numbers following the construction code indicate the rim diameter (in inches), making vital tire-wheel matching easier. After the diameter, the next two numbers represent the load index. The higher the number, the heavier the load the tire can carry. The speed rating, which denotes the maximum speed at which a tire functions properly, is included in the service description, found at the end of the coding sequence. The only exception is a Z-speed rating. If a "Z" is located just prior to the diameter, the tire is Z-speed rated capable of maintaining speeds in excess of 149 miles per hour.
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References
- Photo Credit tires. wheels. image by Cristian Ilie Ionescu from Fotolia.com
