Monoblock II tire size question

[BBad199]

I was considering putting staggered monoblock IIs on my car with a decent summer tire. The trick is, I don't want to mess with the suspension if I can avoid it and I'd like to maintain a good ride quality.

My Idea is:

18x8 AMG Monoblock II ET31 with 235/40/18 Sumitomo HTR Z II
18x9 AMG Monoblock II ET35 with 265/35/18 Sumitomo HTR Z II


I own a black 98 C280 elegance model.


How much of a sacrifice will I be making in ride quality if I went 225 up front and or 255 in back?

Thanks!

[BBad199]

Another worry of mine is the effects of mounting different size wheels...

My considerations are :

1) what will the WEIGHT difference in the wheels do?
2) will my speedometer reading be incorrect?
3) will this also effect my odometer reading?

I want everything to work and I realize the diameter has to be identical to the stock wheel to not throw anything off. (correct me if I'm wrong). I was already told by a mechanic I work with not to bother cause it will mess things up...but I'm sure there is a right way to do this...

I need your recommendations on the right tire size!!!

Thanks
Jon