At that age, even from sitting those tires need to be replaced. Rubber only lasts so long before starting to break down (even when not exposed to direct sunlight).Only issue is my GT-4's due the same thing. previous owner put them on in 2007 (at 13.5k now the care has 15k )and the car just sat in the garage. I spent hours scrubbing and they only got so clean. its like something is bleeding into them heaviest on the outside edge of the white letters.