speedometer difference doesn't really matter, the difference is still not significant enough to make a difference

assume you use the same tyres model as the 225/45/18

when you use 235/45/18, you will feel softer sidewall (bit more comfort) and higher rolling resistance since 235 has more contact with the road, and probably increase your fuel consumption too

225/45 on 8.5 is stretching the sidewall a bit, so for the same tyre you will feel more responsive handling using 225/45, but of course the traction wont be as great compare to 235