What are the signs that it’s time to replace my road/TT bike tires? As back and front tires wear out at a different rate, should I switch the front tire to the back before replacing it?
Some have wear indicators on them. I usually don’t rotate; I just buy a new rear one and then when that wears out, I replace both.
As mentioned, many manufacturers place wear indicators on the tread (when you can’t see the wear dots any longer the tire is worn out). Failing that, when the flat spot on the tread exceeds 1cm wide.
For replacement, just replace each tire when it wears out. No need to rotate a-la car tires. It’s not uncommon to opt for different size tires for front/back (ex. 28mm front/30mm back), which also doesn’t lend itself to rotation.
I just replace mine every year, at least the training tires. My race tires I don’t have to replace.
I just find I get a lot of seriously annoying flats when the tire starts to wear down, and its not even hitting the tread indicators those times. I will add that I ride a lot indoors so I’'m not putting up megamiles on my outdoor tires.
I use Continental 5000S 25mm X 700C tires with TPU tubes. I usually buy 4 or 5 tires at a time to get thru the year. I replace chains every 2,000 miles and have had SRAM cassettes last over 20,000 miles using this method. I, at the same time take the front tire off and on the back it goes with a new tire on the front. So I get 4,000 miles out of a tire unless somehow I trash a tire and that does happen although very infrequently.
Thank you!
How often do you replace the back tire? Every 2,000 miles?
And if I understood that correctly, you then put the back tire in the front?
How about age? I’ve got a set of Gp5000 tires that have done like 200 miles this year but I just haven’t ridden my road bike much. Will they be ok for next year or will they start perishing? I notice GP5000 start getting that cracked appearance after a year even if stored indoors.
When they start looking square and the canvas starts to show. 
The opposite. You want the better tire (newer, more grip) on the front. So this means putting the newer tire on the front, and what was on the front gets moved to the rear.
Oh, wow! I had no idea. I always thought the new tire goes in the back as you put more weight on it. Thanks!
And that’s also why the rear tire tends to wear out faster (more weight), and will get “squared off” sooner. But that also makes tire rotation work well. When you need to replace the rear, move the front to the rear and put the new tire on the front.
Got it. And how do you know when to replace the back tire? I don’t keep track when I hit 2,000 miles. 
If these are the tires you use for racing you do not want it to square off. Typically the front will need to be swapped out every 200-300 for optimal aero benefits
How much stretch are you getting at 2k miles? I’ve been going longer, but wondering if I should be checking more frequently.
This is a difficult question.
Racing tires should be replaced regularly.
Commuting or training tires less regularly.
I started replacing both at the same time.
This actually makes life a lot simpler!!
For commuting/training tires- I keep a bunch of old but acceptable tires around. This way i have a spare, if I get a big puncher in one tire of a “set”.
When I replace my chains I don’t bother to measure the stretch since I know it’s not much and I don’t buy cheap chains. Even expensive chains are way cheaper than having to replace your cassette and rings. I should say ring., I don’t have a one by but it is very flat where I live so I’m in the 53 tooth all the time. The only time I have been in the 39 is to make sure my front derailleur is still working.
I assume chain wear and replacement is also driven by power and style of riding - powering big gears vs. spinning casually probably makes a difference in wear, in addition to amount of miles.
I measure the stretch w/those chain tool thingys. Usually when I clean the bike. I changed my chain about a month ago - when the tool showed stretch. But interestingly, dropping it into some links showed stretch, others links showed no stretch. So my chain wear is uneven, and it’s a good idea to use the tool thing on various sections of the chain when checking.
The tools seem okay. I use a machinist ruler and when I did measure I would measure 12 links. I do long rides but most of my rides are short and fast. Since I don’t beat myself up I do recover from my rides. I do push my chains pretty hard.