Compared to a 245/45R18 fitted to an 8" wide rim, a 225/50R18 fitted to a 7" wide rim would give far more protection from kerbing damage because the tyre's sidewalls bulge out about twice as much...as shown in the chart below. Amount tyre sidewalls bulge out each side from rim 245/45 fitted to 8" wide rim 7.9mm 225/50 fitted to 7" wide rim 15.6mm The reason that the difference is so big, is because the actual width of a 245/45 fitted to an 8" rim width is 243mm, and the actual width of a 225/50 fitted to a 7" rim width is 233mm, ie. the 245/45 is only 10mm wider not 20mm wider...which you might expect. See the chart below. What is happening, it that the sidewalls on tyres with an aspect ratio less than 50 don't tend to bulge out much, but once you get to tyres with an aspect ratio of at least 50 the tyre's sidewalls tend to bulge out a lot more. Hence, if you want good rim protection from kerbing damage choose tyres with an aspect ratio of at least 50 and fit them to relatively narrow rims or at least a rim that isn't wider than normal for that particular tyre size. Actual width of inflated tyre measured from sidewall to sidewall at the widest point 245/45 fitted to 8" rim width 243mm 225/50 fitted to 7" rim width 233mm As you can see in the ETRTO chart below, a 7" rim width is the standard rim width for a 225/50 tyre and an 8" rim width is the standard rim width for a 245/45 tyre...hence both 7.0 and 8.0 are highlighted. ETRTO approved rim widths for passenger car tyres 225/50 6.0-7.0-8.0 245/45 7.5-8.0-9.0