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#26: 06-20-2004, 01:14 PM
outahere's Avatar
Title: HighIntensityDenial
Location: Idaho
Car: '05 Legacy GT Wagon 5MT
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Nearly all tire manufacturers list tires in diameter.
This is true, but they also list static loaded radius, and revolutions per mile.

Radius is different pending tire pressure, where you measure, and other factors.
Diameter measurements also vary with pressure and where you measure.

There is a given distance of tread on the tire and one rotation it covers that distance. Plain and simple.
This is only true for a solid tire with a fixed circumference. Not true for an inflatable tire. A 215/45x17 tire inflated to 10psi will travel a shorter distance in one rotation than the same tire inflated to 40psi, especially when the tire is under a load (i.e.mounted on a vehicle). At 10psi the tire has a smaller rolling radius (and smaller effective circumference) than at 40psi. Plain and simple.

If you want to measure the distance an unloaded tire travels, then use the diameter in your calculations. If you want to measure the distance a loaded tire will travel, then use the static loaded radius in your calculations. Since the tires, when mounted on the car, are in a loaded condition, logic would dictate that the static loaded radius will yield a more accurate calculation.