Not to add to the confusion but here is an example that may help you:
18x8.5 - 235/40/18 - Good size fit, no 'mushroom', no stretch
18x8.5 - 225/40/18 - Still a good fit, but it'll look slightly stretched since it's narrower by 10mm compared to the 235 width.
Also note that different brands have different shoulders which may look different even with the same size. IE: Yokohama S-drives will appear to be wider than say, a Hankook V12.
if you'd like to see different tire sizes on different rim widths.