The Basics of Tire Measurement
Tires being flexible products are affected by many variables, so tire manufacturers use a standard method of measurement. The results of these measurements are the rounded off numbers molded on the tires. In posting these dimensions tires are mounted on what is called the “Design Rim”. The Design Rim is the rim that has been calculated to be the optimum or best all-around rim for a given tire. The tire to be measured is placed on the design rim, inflated to the maximum pressure shown on the sidewall of the tire then laid flat for 24 hours at room temperature. Then the air pressure is rechecked and adjusted if needed, and then the tire is measured circumferentially with a steel tape. The steel tape has to be placed accurately on the center of the tread. This number is divided by the mathematical Pi and the result of this calculation is the number that is posted as the tires overall diameter.
As an example if a tire has a design rim width of 10 inches, and the tire is mounted on a rim that is either wider or narrow than the design rim width two things change. If the rim is wider than that of the design rim the tires overall diameter will be less and then cross section (sidewall to side wall) will be more. If the rim in narrower than the design rim the overall will be greater and the cross-section will be less.
Measurements are never taken with a tire mounted on a vehicle. When mounted on a vehicle two other things come into play which is the loaded and unloaded radius. The unloaded radius is the distance from the center of the axle to the top of the tire and the loaded radius is the distance from the center of the axle to the ground. The unloaded radius will not change, but the loaded radius will change depending on the air pressure in the tire and the weight of the vehicle. A change in the loaded radius affects performance similar to a change in the gear ratio. The vehicle’s ground clearance is also affected by lower air pressure. If a tire is measured from the ground up while mounted on the vehicle the overall diameter will not be in accord with the proper method of measuring and the height of the tire will be affected by the vehicle load and the air pressure in the tire. Because of all these variables, tire manufacturers allow for about a 2% leeway up or down from the markings on a tire. Tires that have been in service for a while tend to slightly grow but then slightly become shorter as wear takes place.
