A vehicle underbody assembly including a pair of rockers, a pair of side rails, a first cross member, a second cross member, and a fuel tank is provided. Each of the pair of side rails is secured to a rearward portion of one of the rockers. The first cross member extends between the side rails and is thermally treated to form a first central hard strength zone between two first soft strength zones. The second cross member is arranged with the pair of side rails and the first cross member to form a rectilinear frame and is thermally treated to form a second central hard strength zone between two second soft strength zones. The fuel tank is secured between the pair of side rails so that the fuel tank is located within a rectilinear frame formed by the pair of side rails and the cross members.