A safety device for suppporting the wheel of a vehicle that has a flat tire. In a first embodiment, two semiwheels are provided which are connected to the wheel hub by a layer of deformable elastomeric material which absorbs the shock that is generated when weight is transferred from the tire to the safety device. Additional support structure is provided that becomes engaged when sufficient weight is placed on the semiwheel. In a second embodiment, a tire is provided with a membrane separating its interior into two independent chambers. The membrane has a circumferential projection for supporting the outer tire tread should the outer chamber suddenly deflate. A novel tool for installing the safety devices of the present invention is also disclosed.