Originally Posted by
Clyde1820
As with tires on the car, I also try to gauge the tires' ability to remain planted on less than ideal surfaces. When minor amounts of gunge and water noticeably reduce the grip, then for me that's the time to replace them ... no matter what other indicators might not exist at that point. For me, it's just not worth failing to have that "normal" grip. On a car, that's one thing, since at least you're somewhat protected in the "cage." But on a bike, it can easily mean a face plant in a ditch or riding into oncoming traffic.
On dry pavement car tires and bike tires generally work better when they're worn. For performance car tires you can actually pay extra to have them shaved so the tread size is reduced.
23mm bike tires don't need tread for riding on pavement so worn tires will handle as well as new, until they flat...