The increase of rubber stiffness with age causes radial tire cornering stiffness to significantly increase during warehouse (shelf) aging. The increase in cornering properties is linear with the logarithm of time and dependent on storage temperature. Five different brands of radial tires studied age in a very similar manner, exhibiting the apparent effect of two different aging mechanisms. A limited experiment indicated an approximate balance of aging and break-in effects on radial tire cornering properties in normal service. The in-service results imply that neither force and moment tests nor vehicle handling tests should be preceded by break-in or scrub-in procedures if the desire is to predict in-service behavior.