Tire size difference and PSM
 

I have a 2001 S with PSM. I can get a deal on a lightly used set of Toyo Proxes 888 where 2 are 225/40/ZR18, 2 are 255/35/ZR18. the 255s are shorter than the 265s I have today. I realized this is a small difference but I don't want to buy them and find out my PSM will trigger too much. Anyone know? (being rear tires, I am thinking PSM might apply the brakes to the rear more than necessary) THanks!

PSM will only trigger more if the tires are sliding more due to the reduced contact patch.
If you had AWD the reduced height might be a problem.

It's the abs that it'll confuse

Word to wise... I wouldn't use R888s for anything other than DE/AX purposes.

I have a set, for such purpose and they perform well BUT on street (I'll throw on only to get to/from local events) they make car sound like the wheel bearing in each corner is cooked...

The 986 may be different, but on my 987 the PSM/Traction Control will kick in all the time if the front tire is much taller than the rear ( vs. OEM ). It sees the increased relative rear wheel speed as wheel-spin.