Anyone know what temp these summer tires start to turn brittle?

 

Do you mean cold temperatures? If so, that is 7C officially. I have driven my RCF in the early days (2017) when I bought it with the OEM Pilot Super Sport tires even when the temperature dropped to 2 C in late fall weather and I was still able to get back home. The tires and bushings in the car became rock hard and car was super twitchy especially on the wet road, but we did safely get home.

 

Thanks, I'd read that it starts around 40 F (about 4.5 in C) but that's road temp. Well traveled roads tend to be warmer than air temp due to traffic (why snow melts when it hits pavement not soil next to it), but I will drive other vehicles when it's colder than 45 F.

 

Correct. The smoother the road, the more dangerous because road imperfections actually help with traction once the tires get really hard.