Drivers can check the depth of their tire tread by sliding a coin into the tire’s circumferential groove. Checking tread depth on an average passenger vehicle can take as little as thirty seconds. In California, this major groove has to be at least: 4/32nd of an inch deep on the steering axle, and 2/32nd of an inch deep on the other axles. To test the depth of the tire treads on the steering axle – usually the front wheels – drivers can use the quarter test.