when am i suppose to change my tires. i've heard i should change them in pairs also. ive got 6400 miles on my bike so far and i havent felt it slip on me yet but there is a noticable void in tread along the center of the tire thats about an inch or two wide. im goin on leave next month and dont wana spend the money unless i have to. should i get new ones now or when? Power to the Buellers!!!