Taking a rectangular vehicle capable of doing this, its turning circle would in fact be equal to the diagonal length of the vehicle. The tightest turning circle possible for a vehicle is the one where the vehicle does not move either forwards or backwards while turning and effectively simply rotates on its own axis. Turning circle refers to a theoretical minimal circle in which a vehicle or boat can be turned around. Turning radius of a passenger car - wall-to-wall radius / diameter at the top, taking the front overhang into consideration, and curb-to-curb radius / diameter at the bottom.