Not really sure why you would use a 245/40 on the rear, if it's an 8.5"" wide rim of course.
My 255/35 Hankook Evo 2's look lovely and thick, sidewall wise, compared to my previous 255/35 vredestein sessantas.
Either way, we need more information.
Are you saying you want info on 255/40/18 or 255/35/18?
What size are your current wheels, width wise and overall diameter?
Why do you currently have 245/40 on them?