Tires are comprised of many layers of rubber, steel and fabric. Due to these different components, your new tires require a "break-in" period to ensure that they deliver their normal ride quality and maximum performance. As tires are cured, a "release lubricant" is applied to prevent them from sticking in their mold. Some of the lubricant stays on the surface of your tires, reducing traction until it is worn away. Five hundred miles of easy acceleration, cornering and braking will allow the mold release lubricant to wear off, allowing the other tire components to begin working together. It is also important to note that your old tires probably had very little tread depth remaining when you felt it was time to replace them. As any autocrosses or racer who has tread rubber shaved off of his tires will tell you "Low tread depth tires respond quicker." Don't be surprised if your new tires are a little slower to respond (even if you use the exact same tire as before). Their new, full depth brings with it a little more tread squirm until they wear down.