To check if a tire meets legal minimum tread, place a penny upside down in a grove on the tire. If the tire tread covers any part of Lincoln's head, the tire meets minimum standards. Most state laws require 2/32 inches of tread.
Alternatively, test your tires for legal minimum tread by visually checking for wear bars. The Federal Motor Vehicle Safety Standards requires all manufactured tires to have wear bars. Wear bars run perpendicular to the tread pattern near the bottom of the grooves from the inside to the outside shoulder of the tire. Wear bars only become visible when a tire no longer meets minimum requirements. Alternatively, use a tread-depth gauge to measure your tires.
Though 2/32 inches of tread depth is required, experts recommend 4/32 inches to prevent hydroplaning and losing traction in rainy weather. To determine if a tire has 4/32 inches of tread, place a quarter upside down in any grove. If the tire tread covers any part of Washington's head, the tread depth is more than 4/32 of an inch. For snowy weather, experts recommend 5/32 inches of tread. To measure for this depth, place a penny upside down in a grove; if tread covers the top of the Lincoln Memorial, the tire has approximately 6/32 inches of tread depth. Some winter tires have a second set of wear bars at 6/32 inches of tread.