Hello BF.
Was wondering, if there are well defined indicators to determine the correct time for changing my road bike tyres?
Specific age? mileage? any visual changes? recurrent punctures?
xx
Carsten

You can see threads or the tube itself.

If it's a treaded tyre, loss of tread. If not, obvious signs of wear, flattish patches. In extreme cases, visible thread casing. Recurrent punctures can be an indicator, while you have the tyre off the bike check how thin it has become.

Yes, all true, and certain brand/models have specific indicators like the two depressed dots on Continental Grand Prix 4000. When the dots disappear (tire is worn level with the bottom of the dots), Conti says it is time to replace. I have never gone that far, Recurrent flats have always encouraged me to change the tire sooner. Of course, YMMV.

Probably only 1/2 worn then.

IMO, manufacturers put wear indicators on tires to get you to buy tires more frequently than you you otherwise would and more frequently that needed. Replace a tire when the cords start showing.

In my experience, worn tires flat no more frequently than new ones, but his may depend on the types and number of tire hazards where you ride.

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.

^ ^ I have never noticed a difference in how a tire rides or handles as it wears.

I ride them to the cords as others have mentioned.

I don't doubt you're right. It is a small savings to keep riding the tire past the indicator. I wouldn't be in a panic to change the tire right at the indication point, but it couldn't hurt either. Cords? I dunno. That is a little extreme for my taste. I feel like it is just asking for trouble.

Flatting often.

Wear indicators are like pressure ratings: take 'em with a grain of salt. As stated above, the manufacturer is going to err on the side of increased sales and reduced product liability.

Personally, I'd rather change tires before I start to get more punctures, because changing flats is an inconvenience. But this is a function of your disposable income.

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...

Perhaps. My gauge, though, is when they begin to clearly lose grip on the varied surfaces I typically ride on. Pretty good gauge, I've found, irrespective of tire type.

The guy that works the bike counter at my lbs (an outdoor supply company, tents, boots, etc...) says he changes his every 3 months, tires and tubes, just to be sure. "Your tires are looking a little squared off, I'd probably go ahead and replace them". They were Lithion 2s that had about 75 miles on them. He also doesn't trust patches. Ever.
I ride em until start having flats, or i see a cord. Move front to back, new tire on front, repeat ad infinitum.

I have had the conti's with wear indicators and never really got to the bottom of the little dots. Usually replace after 2500 miles or so any way. I hate stopping to fix flats.