The two-digit number after the slash mark in a tire size is the aspect ratio. For example, in a size P215/65 R15 tire, the 65 means that the height is equal to 65% of the tire's width. The bigger the aspect ratio, the bigger the tire's sidewall will be.
How to read tire size in inches is very straight forward. Example A : 33X12.50R15 The first number is your overall diameter of the tire. In this example the tire diameter or "height" is 33 inches tall.
ST = If a tire size begins with "ST," it signifies the tire is a "Special Trailer Service" size that was designed to only be used on boat, car or utility trailers. ST-sized tires should never be used on cars, vans or light trucks.
The first three digits indicate the section width, or cross section, of the tire in millimeters. The section width is defined as the widest point of the inside sidewall to the widest point of the outside sidewall when mounted on an appropriate wheel . For this ZE950 tire, the section width is 245 millimeters or 9.6 inches.
The load index and speed rating, or service description, are the numbers that follow the tire size. The load index tells you how much weight the tire can support when properly inflated. Load indices range from 75 - 105 for passenger tires, with each numeric value corresponding to a certain carrying capacity.
According to tirerack.com, some tires produced in the 1990s may have a small triangle following the DOT number to identify the decade. But any tire that has a three-digit code is history.
If you already know your tire size and would like to find the right Bridgestone tire for your vehicle, visit our Tire Catalog. While actual, physical measurements like width and diameter are factors in finding the right tire size, there are additional details you should consider.
Tire size was specified as the tire width in inches and the diameter in inches - for example 6.50-15.  From 1965 to the early 70's, tires were made to an 80% aspect ratio.
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There are two ways that tire manufacturers display tire size, inches and metric. How to read tire size in inches is very straight forward. Example A 33X12.50R15 The first number is your overall diameter of the tire..
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