Farthest distance on cylinder

The farthest distance between two points on a cylinder is the diagonal distance between opposite ends of the cylinder.

Diameter $d = 2 \times 8 = 16$. Height $h = 8$.

Farthest distance = $\sqrt{h^2 + d^2} = \sqrt{8^2 + 16^2} = \sqrt{64 + 256} = \sqrt{320} = 8\sqrt{5}$.