Even tire wear. monitoring every few months. Measure in these units. Keep a diary. Know when to change tires or change a pair. 2/32 inch is the acceptable wear limit, but only a fool rides on bare tires. For your personal safety as well as your passengers and other people on public roads; Replace worn tires with the remaining 4/32" to stay safe and avoid aquaplaning in the rain. Avoid ditches in winter. Always keep the device in the car. Or being cheap and ignorant, sitting in the ditch and leading the tow truck operator and body shop to success. Knowledge is power. Be a smart cookie.