Drivers in the market for new car tires will be asked by the auto mechanic what size tire they need. To answer this question, the driver must inspect their tire sidewall which displays a long string of numbers and letters.
While seemingly incomprehensible, this code provides important information about the tire's dimensions, weight restrictions, intended purpose, internal construction, and high-speed durability.
The Six Categories of Tire Measurements
Car tires are measured in six categories: service type, section width, sidewall aspect ratio, internal construction, tire and wheel diameter, and service description. Each criteria explains how the tire was manufactured and how it is to be used. The example tire size featured in this article is P195-70-R14 90T.
- Service Type. Tire sizes start with a letter that identifies the type of vehicle and/or type of service for which the tire was designed. In the example of the P195-70-R14 90T tire size, P means passenger for minivans, SUVs, light trucks, and cars. Other tire sizes include T (temporary spare), LT (light truck for larger vehicles), and ST (special trailer for boat or car trailers).
- Section Width. The three numbers after the first letter signify the tire's width in millimeters. In the example of the P195-70-R14 90T size tire, that's 195 millimeters measuring across widthwise from the outer sidewall to the inner sidewall.
- Sidewall Aspect Ratio. The two numbers after the section width identify the tire's aspect ratio, or the relationship of height to section width. In the example of a P195-70-R14 90T tire size, 70 indicates that the sidewall height (from rim to tread) is 70 percent of its section width (also referred to as a tire’s series). Higher numbers mean taller sidewalls and softer ride, and lower numbers mean shorter sidewalls, wider tire, and quicker response.
- Internal Construction. The letter after the two numbers representing the sidewall aspect ratio designates the internal construction of the tire. In the example of the P195-70-R14 90T tire size, R means radial, the most common tire construction.
- Tire and Wheel Diameter. The two numbers after the letter identifying the internal construction indicate the diameter of the wheel in inches. In the example of the P195-70-R14 90T tire size, tire and wheel diameter is 14 inches.
- Service Description. The last set of numbers followed by a letter indicate the tire's load carrying capacity index (maximum weight on the tires) and speed rating (how fast the tires are designed to go). In the example of the P195-70-R14 90T tire size, 90 equals 1323 pounds max (600 kilograms) and T equals 118 MPH max (190 km/h) for passenger cars.
The Bottom Line on Tire Measurements
The tire’s information code may appear confusing, but it informs consumers and auto mechanics as to how the tire was manufactured and its principal use.
Once the driver learns how to read tire sizes according to service type, section width, sidewall aspect ratio, internal construction, tire and wheel diameter, and service description, he or she can gain a greater appreciation for the science of tire construction.
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