also: トレッドウエアインジケーター
noun
tread wear indicator
A raised bar or mark inside a tire's tread grooves that becomes visible when the tread is worn down, indicating the tire should be replaced.
タイヤのトレッドウェアインジケーターが出てきたら交換時期です。
When the tread wear indicator appears on your tire, it's time to replace it.
Borrowed from English 'tread wear indicator'.