I bought some 18for my 1997 540i,what tire size should I go with,I was thinking 235 40 18 all around,tell me what you think? Thanks

depends on the size of the rim...
do you have 7, 8, or 9 x 18...
are th e front different from the back?

I think the default size for 18" rims is 245/45/18 or 245/40/18. Not 100% though, but it depends on whether or not your rims are staggered.

Just to add on.. I have 12X 18 in the back of my M5.. 285/30/18... last year i had 295/30....