Might be stating the obvious (to some) but something I didn't realise until I started trying to pick tyres for aftermarket wheels is that the profile figure - e.g. 40 - is 40% of the width.
So for a 255/40/17 tyre - diameter is 255mm, profile / side wall is 102mm (40% of 255), and radius is 17inches.
Learning this made it all make a lot more sense to me. I also have this graphic to refer to. Can't recall where I found it but it's now saved on my desktop!
I also wanted a chunky looking tyre with a solid side wall and after going 245(F) / 265 (R) , have since switched back to 235(F) / 255 (R) as I found the handling was noticeable more boggy and I use the car for fast road and track use. For ref, these tyres are on 18" BBS LMs.
By
sumshiftyguy ·
So which is best? Auto or manual
Sorry guyZ, couldn't help myself 
CheerZ,
Andy