You are actually calculating "arc length." The formula for that is s=r times theta,
where s is the arc length or distance traveled in a circle, and theta is the central angle (always in radians, never in degrees).
Actually, you could obtain the same result by using the formula for circumference of a circle. The distance around the wheel is 2 pi r, where r is the radius and is one half of the given diameter. calculate this circumference / distance, and then multiply your result by 2.
Note that your final answer MUST include units of measurement (of distance).