when to replace tires tread depth
1 Replace the tire if the top of Abe's head becomes uncovered by any of the grooves.  If your tires are only partially worn, find out how much. Stick a tread-depth gauge (about $5 at auto-parts stores) into each of the grooves on all four tires, as shown in the photo below. Then check how many inches of usable tread remain and match your driving to the tread wear.