What brand and model of tire is it? I work in a tire shop and we sold a large number of Uniroyal tires that were self-sealing (supposedly). The model names included Royal Seal and NailGard. I think General also sold a line called GenSeal. These tires have a layer of black sealer inside, mostly across the tread area of the tire. The stuff looks almost like roof tar, and is extremely difficult to remove. If it is one of the self-sealing tires, they have a tendency to go out of balance after being subjected to extreme heat.