245/40/18 on 18x8?
 

Will that work, or is it too wide.

Also, is there any advantage of going with a 35series tire over a 40? Does 40 offer a substantial amount of protection over the 35, or are they just more cushy?

Thanks.

It would fit the wheel, but you would actually want 245/35/18.
 

Also, that size may not fit without rolling fenders (if you haven')

Mikeyb in MNAudi has 245/35/18 on his ET35 18" wheels and he's not even that low and it rubs.

Oh, and rim width for 245/35/18 is recommended from 8" to 9.5"