Originally Posted by
espuma
Nope, radius is tied to circumference only for a circle. Once you put the tire on the ground it is no longer a circle.
I agree with njkayaker. As the wheel rolls the distance traveled is the effective circumference of circumscribed by the radius from the center of the wheel's axis and the weighted wheel's contact patch on the road. On a smooth road this is a perfect circle, 2 r pi, where r is the effective (weighted radius).