When does everyone switch from training tires to race tires? I was planning on switching on Sunday for IMMT. That gives me a chance for maybe a short ride to two to get the feel of them, and make sure everything is all good to go.
A couple of weeks before at least
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Absolutely.
In my experience at least 50mi to check they’re installed ok and scrub them in. Crr should also come down a bit from new.
As ever, nothing new on race day! 
A couple of weeks before at least
Nooooooo. Your tires are screamin’ at you for losing their aero properties 
When does everyone switch from training tires to race tires? I was planning on switching on Sunday for IMMT. That gives me a chance for maybe a short ride to two to get the feel of them, and make sure everything is all good to go.
I do a couple of things, usually I like to put them on Friday or Saturday before more morning rides. I have done after the Saturday morning ride as well, but that generally isn’t my recommendation unless confidence level is high. Put on the new tires just early enough to get a few miles and make sure they are installed correctly and don’t have an early cycle failure in them, but not too much. Tires lose their aeroness as you use them. This is more of a concern for the front than the rear but given I raced Supersonics for so many years and you don’t get than many miles anyway I was replacing the rear quite frequently.
Better think about what type of road surface you are going to be taking those short
2 or 3 rides on. Put my race tires on and rode them 3 days before my last race. They
were cuts everywhere and they trashed. I didn’t have experience riding on crap roads
and didn’t know how much they could chew up a low durability tire.
Hardly ever had a flat with my conti tt, so would see no reason to not ride them in training.
So my answer is never, because I’ve got them always on.
race tires stay on race wheels. race wheels went on when I was packing to go to the race. A quick derailleur adjustment, if necessary, and everything is good to go.
The first time I mount them maybe, maybe, 2 rides before the race. And that’s usually an hour then 30 minutes the day before. Hour ride is on trainer and 30 minutes usually is too depending on where I’m racing.
After first time I race on new tires I mount them the day before and before that 30 minute ride to make sure they’re mounted correctly and the latex tube is good to go.
I see no need to ride them too much because then you are just wasting its lifespan and raceDAY tires already have a short lifespan as it is.