[ChrisZang]

my next 2 track days are coming up early August (Thunderhill).

I track on my street tires (Yokohama AVS 100) and I currently have about 2.5 millimeters left on the rears and 3.5 on the fronts. I'd guess that the track days will cost me about 1 ~ 1.5 millimeters. That means that after the track days these tires will be finished.
So now I am wondering if I should put on new tires before going to the track (and partially ruin them) or "finish the old ones" on the track and then get new ones.
What's the general consensus here:
Is it better to track on old (broken in) tires or on new ones?

By the way: As I live in CA I will not encounter any rain for at least another four months, so hydroplaning is not a factor in my decision.