It is because the temps are below 10*C the Bridgestone RE050 specs say they have less traction in the cold weather specifically under 10*C. The tires will harden up in the cold weather causing you to have less grip, in the summer they will heat up and get nice and sticky giving you more grip. I would recommend that you inspect the tires tread depth though to because I know that mine (which need replacing) have gotten progressively worse as the tread depth got less.
I am almost at slicks on my rear and can't wait to get new ones in the spring!
Congrats on the new Z! enjoy, also hope to see you at some GTA meets next year seeing that your in Burlington.
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