can i fit 245/40/18?

[al 350i]

i've been offered a pair of conti sport 245/40/18 at a really good price which i was thinking of mounting on my spider wheels for the rear of the cerb but i'm thinking that they might be to big (diameter 653mm as oppossed to 636mm for a 255/35/18), has anyone fitted this size before? also would i loose much grip as compared to 255's (although should gain a better ride).