I'm also on GR86.org, so I'll just paste my comment from there:
215 stock MP4 vs 225 MP4
S I have no idea if it's being more gripper model, or wider tyre, but Michelin 215 is realitveily narrow, while 225 is bulky as hell. It almost feels like jumping to 235. From the looks perspective, they look more serious, raw, much better although it's only "a step up". Unfortunately you loss some of the precise and refinement with wider wheel. It's a trade off, slightly more grip vs less input feedback. They feel a little floaty when pushing hard in turns. 225 is the way to go for 8J, but you can run them on stock 7,5J. Just my feedback is that they are less balanced than stock width.
I do appreciate the positive factor, that they are audible less noisy, probably due to different compound being more soft. A nice bonus.
This was from original post.
After that I did more than 10k kms on these and can fully repeat myself.
Simple formula - 225 tyre goes to 8J, 245 on 9J and so on. The "middle" sizes such as 235, you guessed it, 8,5J.
More stretch = better precision.
It's floaty enough for me to reconsider going back to 215 or swapping the wheels to 8J...
View attachment 31265
View attachment 31264
View attachment 31266
View attachment 31267