Performance Tire and Wheel Relationship
Tire and Rim Association (TRA) Yearbook
To maintain consistency in specifying a tire's measurements, representatives of all major tire companies and most minor brands formed The Tire and Rim Association (TRA) in the United States. The TRA has established technical specifications that are based on engineering principles and approved practices. Every year they publish the TRA Yearbook, which contains all TRA standards and related information approved by the Association. These standards include:
- Tire Designations
- Tire Load Ratings
- Rim Widths
- The Design Rim or Measuring Rim
Standards for Metric tires are established through a similar association called the European Tire and Rim Technical Organization (ETRTO). The approved rim width range for each tire size has also been carefully selected by the TRA and ETRTO organizations. For example, the P255/50VR16 tire is approved to be mounted on wheels ranging from 6-1/2" to 10". Past experience and engineering practice has shown that wheel widths outside these ranges will stress the tire in a manner that may result in poor service and potential tire failure.
It is important to note that there are some differences between P-Metric and Metric approved rim width ranges (in general, Metric tires have a slightly narrower range). The TRA also has developed a measuring rim for each tire size that enables all manufacturers to measure their tires on the same size wheel. For 50-series tires and higher, the measuring rim width is 70% of the tire's section width rounded off to the nearest 0.5." For example, a P255/50VR16 tire, which has a design section width of 10.04", is measured on a 7" wide wheel.
Mid-Range Rim Widths
Provide a balance between handling capabilities and ride quality. The wheel's width influences handling and ride quality. Always choose a rim width within the range of the tire's acceptable rim width specification.
For tires that have an aspect ratio lower than a 50-series, the measuring rim is 85% of the tire's section design width. Therefore, a 255/40ZR17 tire with the same 10.04" section width would be measured on an 8.5" wide wheel. The measuring rims for Yokohama tires are listed with their dimensional data in this guide.
As mentioned previously, the actual dimensions of a tire are dependent on the rim on which it is mounted. The dimension that changes the most is the tire's section width—a change of about 0.2" for every 0.5" change in rim width.
For an A032R in the P255/50VR16 size, Yokohama's dimensional data indicates a section width of 10.2", an overall diameter of 26.0", and a tread width of 9.1" when mounted on a 7.0" wide wheel.
While the tire size designation indicates that this is a 50-series tire, let's see how the tire's aspect ratio is calculated. A tire's aspect ratio is the relationship between its section height and section width. Since the section height is not provided, we must calculate it.
Determine the section height.
Results in an improvement in ride quality, but may sacrifice some of the tire's ultimate performance capabilities.
Subtract the rim diameter from the overall tire diameter and divide by 2 as shown in the following formula:
(Overall Diameter - Rim Diameter) Ö 2 = Section Height
The graphic to the right shows how to determine the section height of a P255/50VR16 tire.
More than one dimension may vary with a change in rim width; however, the Yokohama tire dimensions listed in this guide are accurate for the measuring rim widths specified.