The minimum legal tread depth for motor-vehicle tires typically ranges between 2/32 of an inch and 1/8 (4/32). Most drivers have no way of telling if their tires are in compliance with the law, however. The S448 tire-depth gauge from Milton Industries will indicate wear as well as possible alignment problems once you measure the tread depth on the inner and outer grooves of each tire (if they’re not equal, you have a problem). The gauge can also indicate whether the tire has been over or underinflated by measuring tread depth in the middle of the tire. The S448 is small enough carry in your pocket and fits between the narrowest tread-pattern spaces. Pick one up at your local auto parts outlet or from www.amazon.com for about $7.