Originally Posted by spclark

To my mind it's kinda like calculating ERD when building wheels. Hard to measure directly w/o some sophisticated tools but the concept is fundamental to getting spoke length right.

With tires on wheels, when tires aren't loaded the effective circumference is less than when the tire is loaded. It's not the distance traveled so much as the lever arm of the wheel's effective radius when loaded that affects calculations with that variable plugged in.