How Tire Measurements Are Calculated
Tires are flexible products affected by many variables, so tire manufacturers use a standard method of measurement. The resulting measurements are the rounded numbers molded onto the tire sidewall.
The design-rim measurement method
To establish tire dimensions, manufacturers mount the tire on what is called a design rim. The design rim is calculated to be the optimum all-around rim width for a particular tire.
The tire is mounted on the design rim, inflated to the maximum pressure shown on the sidewall, and left flat for 24 hours at room temperature. The air pressure is then rechecked and adjusted if needed. Next, the tire is measured around its circumference using a steel tape placed accurately at the center of the tread.
That circumference measurement is divided by mathematical Pi (π). The result is the tire’s published overall diameter.
How wheel width affects tire dimensions
As an example, if a tire has a 10″ design-rim width but is mounted on a wheel that is wider or narrower than 10″, two dimensions change.
- Wider than the design rim: the tire’s overall diameter becomes smaller, while its cross-section width (sidewall to sidewall) becomes wider.
- Narrower than the design rim: the tire’s overall diameter becomes greater, while its cross-section width becomes narrower.
Why tire measurements are not taken on the vehicle
Tire measurements are never taken with a tire mounted on a vehicle. Once the tire is on the vehicle, two additional measurements come into play: unloaded radius and loaded radius.
- Unloaded radius: the distance from the center of the axle to the top of the tire.
- Loaded radius: the distance from the center of the axle to the ground.
The unloaded radius does not change, but the loaded radius changes based on tire pressure and vehicle weight. A change in loaded radius affects performance in a way similar to a change in gear ratio. Vehicle ground clearance is also affected by lower tire pressure.
If a tire is measured from the ground up while mounted on a vehicle, the result will not match the proper overall-diameter measurement. The observed height will be affected by vehicle load and tire pressure. Because of these variables, tire manufacturers allow approximately 2% leeway above or below the dimensions marked on a tire. Tires that have been in service for a while can also grow slightly, then become slightly shorter as tread wear occurs.
Information supplied by Interco Tire Corp.