The issue is not only remaining tread but age of the tires and how they were stored. I bought my car in '04 and it had a set of Hoosiers on it that had plenty of tread but the car and tires had set up for over a year. I was excited about the ex-SpeedSource RX-7 that I had paid the most I ever had for a racecar. I ran the first test day and Sat. race on the old tires and I was extremely unhappy. The car was loose and twitchy and just did not feel good at all. I ran about the slowest times here at my home track that I ever had and was laboring back in the pack w/ the SMs. My purchase was not looking so good. The next day I put on a set of new Hoosiers and - WOW! The car was a dream and I did the best lap I ever had here.

Moral of story - use those tires for practice and inconsequential sessions and get yourself a new set for when it counts. Welcome back, John.