It will make a small difference. Remeber the tire size is a ratio, so 235/40/18 means the tire is
40% of 235 = sidewall height
18 is the rim diameter.
So the sidewall on a 235 is slightly taller than a 225. Also a 225 on a 8.5" rim has a slight stretch to it.
I have 225/40/18 on my fronts and 245/40/18 on my rears (in the process of getting new tires) wish I had gone 235 instead of 225.