I had a link to a youtube video in which Gerhard Berger talked about running different tire compounds on the front and rear of his car after winning his first GP at the 1986 Mexican GP. Unfortunately, the youtube user's account that posted that video was terminated. So it was definitely allowed in 1986 but I don't know when it was no longer allowed. BTW, I really enjoy this mod. Thanks for all your hard work. It was the first year that I started watching F1 on TV.