Some ways to soften tires are wrapping kerosene soaked rags around the tire, painting a mixture of paint thinner and diesel fuel onto the tires or soaking the tires in WD-40. The WD-40 has the quickest results but only softens the tires somewhat.
For the kerosene wrap, soak the tires in water. Then, wrap the kerosene-soaked rags around the tire, followed by a layer of aluminum foil. Keep the tires wrapped for two to three days for best results. Repeat this process every week to keep the tires softened. Another way to soften tires is to mix together equal parts paint thinner and diesel fuel. Use a paint roller to apply two coats of the mixture to the tires.