A deflation-proof pneumatic tire comprises a casing and a substantially void-free, preferably oil-containing elastomeric filling material. The tire casing is injected with a combination comprising a polyol, an organic polyisocyanate and water, in amounts and under conditions sufficient to produce carbon dioxide and form a polyurea-containing polyurethane elastomer in which the carbon dioxide is dissolved in the elastomer, thereby providing a substantially void-free filling material. The formulation permits the addition of a substantial amount of extender oil while retaining an acceptable level of hardness.