A safety tire and wheel assembly which includes an improved multiple-piece wheel rim assembly for preventing inflation and/or venting to atmosphere when the rim assembly components are improperly assembled, and an improved safety insert mounted on the wheel rim base for supporting the tire tread during operation following loss of inflation pressure. The wheel rim assembly includes telescoping rim segments and an O-ring normally captured in compression between opposing portions of the rim segments for sealing the tire cavity. Air bleed passages are formed in one rim segment adjacent the sealing ring externally of the cavity to vent the tire cavity to atmosphere when sealing compressive stresses are absent from the O-ring as in the event of improper assembly or disassembly. The safety insert includes a ledge normally disposed within the tire cavity and having a radially facing surface roughened by a circumferential series of axially extending splines. Encapsulated lubricant is carried by the insert in a position to be fractured by the tire tread upon loss of inflation pressure so that the lubricant is spread over the tread-engaging insert surface and retained adjacent the surface between the splines.