From everything I can tell 4" was it for the 8" width wheel. Im in the middle of determining what size slick to run at the track. Tried a Hoosier 28x9.0x15 on a 15x8 rally. Looks like it will rub on the quarter panel under a good load/suspension compression. The tire has a section width of 11.0" and is just too big. I then took numerous measurements today and looks like I'll have to go with a Mickey Thompson 26x8.5x15 on a 15x7 rally. They have a section width of 9.5" which will give me sufficient margin on each side. That slick should still hook better than the 215/65 BFG TA's! Please excuse the rambling.