A Ferris wheel is a periodic function you can ride: a rider's height traces a perfect sinusoid, and behind that curve is plain circular motion. One revolution covers one circumference, $2\pi r$, in one period $T$, so the rim's speed is $v = \dfrac{2\pi r}{T}$ — read aloud: v equals two pi r over T. It is $v = \dfrac{d}{t}$ wearing formal dress — the distance just happens to be a lap.

The resident trap is the missing $\pi$: $2r$ is the diameter, a straight line THROUGH the wheel, and no rider travels that way. The rim rides the full $2\pi r$ — about three diameters and change per lap, which is a fine mental check: a 20 m-radius wheel serves up roughly 126 m of scenery per revolution.