Driving on worn tires is a safety hazard, drastically increasing braking distances on wet roads. A tread depth gauge measures the depth of tire grooves, usually in 32nds of an inch or millimeters. You place the flat base of the gauge across the tread block and push the metal probe down into the lowest groove. Tires are legally worn out at 2/32 of an inch, but they lose wet traction long before that. Check multiple spots across the tire to spot uneven wear from alignment issues.
Mechanic Glossary
Tread Depth Gauge
A small measuring tool used to determine the remaining depth of a tire's tread grooves.
Frequently Asked Questions
Most tire experts recommend replacing tires when they reach 4/32 of an inch for wet weather safety, and 2/32 is the absolute legal minimum.
This is a classic sign of over-inflation. The tire is ballooning, causing only the center of the tread to contact the road.
You can use the penny test. Insert a penny into the tread with Lincoln's head facing down. If you can see the top of Lincoln's head, the tire needs to be replaced.