Tyre size options for 18 x 9 inch rims

[matina]

Just bought an M3 evo. I have to change the rears as they are getting low.


Ive got 8.5 x 18 inch BBS split rims on the front. They have 225/40/18 dunlops

This is so they can clear my AP brakes, although 8 inch is sufficient.

My rears are 255/35/18 on a 9 inch rim.

My question is.

Can I fit a less wider tyre with bigger side profile say, 40 to keep the rolling circumfrence the same and costs

and what tyre sizes would be ok on the rear 9 inch rims.

Would a 235/40/18 fit?

All help would be appreciated.