The standard size for 981 S is 235/265 45 19 but on the track you want to increase contact patch. Many of those I spoke with use 18" rims to save on tire cost but there are not many combinations out there. I was told of this interesting option. Use square set up of all 4 tires at 245, 255, or 265 with aspect ratio of 35 or 40 on an 18" rim, then put a 20-30MM spacer in the rear. Assume PSM off or on based on your skill level.