A unitary run-flat insert for inflatable tires is provided in the form of a split ring having sufficient flexibility, through separation at the split portion thereof, to pass over the rim flanges of a tire mounting rim and having an inner periphery closely and circumferentially engaging the rim when the split portion is closed, with lock means at said split portion adapted for automatic engagement as the ends forming said split portion are moved into juxtapositioned alignment. As closed by the lock means, the insert engages the rim in a manner to rotate therewith in normal operation, but to slip circumferentially thereof when riding on a deflated tire. The flexibility required can be provided by fashioning the insert from material having sufficient deformability to permit flexing of the full cross-section of the insert or, when less deformable material is needed for load bearing purposes, the insert can be provided with a plurality of slits, circumferentially spaced and extending part way through the radial dimension. The unitary insert can have a substantial radial dimension when intended for load bearing under run-flat conditions, or can have a smaller, non-load bearing radial dimension when intended primarily to maintain bead-seating between the tire and rim under run-flat conditions, thereby facilitating the use of standard, one-piece drop center wheels with tires especially designed for "run-flat operation", and which provide their own load bearing support when running flat.