Although a quick visual inspection is what we usually settle for, there are times when a closer inspection of tread wear is required.
THE PENNY TEST Select a portion of the tire where the tread seems lowest. Take out a penny and insert it into the tread, head sign down.
If any part of Abe's head is covered by the tread, you're driving with the legal and safe amount of tread. If your tread gets below that (approximately 1/16 of an inch), your car's ability to grip the road is greatly reduced, particularly in rain or snow.